I'm about to buy myself a new bike(its between the trek 7.2FX and the 7.3FX) and I was wondering what are the key accessories that every newbie just getting into biking/commuting should get? I am on a little bit of a budget so I don't to get suckered into anything I dont really need.

Thanks!

Frame mounted pump, patch/tire kit with an extra tube, fenders are a MUST if you are going to ride in the rain at all, and a helmet. I'd say get a rack of some sort if your commute is over 2-3 miles as well, because a backpack on your back is going to suck. Save until you can get all of these things and you will be better off.

EDIT: And if you are going to ride at night at all, you need at the very least a rear and front light set. I was coming home the other night, and some kid on an MTB rode past me, no lights, nothing reflective at all. Great way to end up as a human pancake.

I'd consider these essentials. Rest are a matter of personal preference. I commute 56+ miles per day, run no fenders and wear a messenger bag for example.
Add a solid small tool kit on top of the patch/tire kit and some basic mechanical knowledge.

Oh, and don't cheap out on the lights if you're riding a lot at night. You want to be seen

You'll probably want a bag of some sort, maybe more than one. I have a seatpost bag which can no longer hold all I want to carry, so I am getting some tube panniers. Storage space is great for your bike tools and spare tubes, as well as your personal stuff you need that you may not want to pocket carry. Or to carry your lunch.
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Essential? A multi-tool, patch kit, spare inner tube, pump, and lights (if you ride at night).

There are a few other things that are not strictly essential, but definitely in the nice to have category. I would include things like a rear rack, panniers/trunk bag, water bottles and cages, a rear view mirror, and of course fenders. Some would also argue that a helmet should be included in this category as well.

In addition to a tire pump, patch kits, and the tire levers, be sure to get one of those compact tool kits. Something is going to loosen up and you are going to wish you had an Allen wrench. Small first aid kit will come in handy too. Get a $17 combo headlight/taillight, you will need them in traffic.

Pump, patch kit, tire lever(s), spare tube, multi tool*, and a seat wedge or frame bag to put that stuff into- that's the bare bones for anyone riding. Then next up would be based upon your riding environment- hydration, lighting, lock(s). Everything else will be personal preference derived through trial and error.

*I use the Park MT-1. Anything I can't do with this with my limited wrenching skills is best left to the LBS.

**Also would say either bus/cab $ or cell phone ranks up there as well...

Bike, lock, front and rear light, small pump and 1 or 2 spare tubes (I just replace the tubes and patch them when I get home, much quicker that way, and the patched tube becomes my nest replacement), a little bike tool kit, plastic tire levers and a bag, backpack, or messenger bag to put them in. Anything else is just a nice extra.

1. The only essential commuting requirement is a job (or school) to ride to and from.
2. I'd learn to change a tire tube before you actually need to.
3. The rest you can gradually fill in, as your preferences become clearer with experience.

I always seem to run up a few hundred bucks for extras every time I get N+1. The extras include:
Spare tube (essential)
Patch kit (essential)
Tire "irons" (essential)
Seat bag to hold the above (essential)
"Frame" pump (essential, currently all Topeak Road Morphs)
Multi-tool (nice)
Fenders (essential for rain commutes)
Saddle (Brooks B-17, essential for me, maybe nice for you)
Saddle cover (essential for the Brooks)
Blinky butts (essential)
Spare headlight (nice)
Computer (nice)

No wonder I can't afford a new bike!

1. Desire to ride a bike to/from work/school daily
2. functioning bike
3. A lock or secure place for your bike (so you can keep it)
4. Knowledge of how to change a flat
5. Flat changing tools
6. Cell phone & cab fare to get home for anything more serious than a flat tire.

Get a floor pump with a gauge to use at home. Get some chain lube and degreaser also a brush to clean the gears and chain once in a while (save some rags), all that will help to keep the chain in good shape. Never lay the bike down on the deraiuler.

If you've never fixed a flat, ask the folks at the bike shop to demonstrate how to do this, learning on the side of the road sucks.

Essentials are things that will assure your safety and ability to finish the ride both ways under normal circumstances. That would be: lights (unless you never ride in the dark), a bell or a horn, spare tubes, patch kit (if you run out of spare tubes), and a pump and a helmet.

Then you have things that would make your ride more pleasant and easier: that's where I'd put a water bottle, a lock, fenders, racks, etc.

You should also have some stuff at home, as mentioned above, at least a floor pump, chain lube, chain/cog brush, some degreaser to clean the bike. And yeah, definitely learn how to fix a flat

You have everything you need,a bicycle and an itch.

As you ride more,you'll need a way to fix flats,maybe a lock.If you ride at night,you'll need at least a couple blinkies.Short of that,you need nothing.

Everything else everyone says is not really needed.It's just gravy.

You don't even need to fix a flat if you can get home walking or on public trans.

The bare essential requirements are somewhere to go and a machine with 2 or sometimes 3 wheels, propelled by pedals. For riding at night or in poor visibility, effective head and tail lights are a must. Some people use lights during the day and/or reflective vests etc to increase visibility. Having good fenders helps significantly when the road is wet, even when it's not raining- surface water gets you very wet and is a lot dirtier than rain water. Unless you have a secure place to store the bike at your destination, a sturdy lock is unfortunately also an essential.

It's sensible to carry a few basic tools in case of problems. I would carry a spare inner tube as well as a patch kit. Changing a tube is far easier and quicker than repairing a flat at the side of the road, in the rain/dark. You can patch the old tube while you're at work or when you get home. Don't forget a mini-pump or the spare tube and patches will be useless. An adjustable spanner, screwdriver and a couple of allen keys can get you out of a lot of difficulty. These basic tools and spares don't take up much space are quite cheap.

Helmets are a big can-o-worms. Some people won't get on a bike without one, others see no compelling reason to wear them. Unless there's a mandatory helmet law where you live, it's entirely up to you and you shouldn't feel pressured one way or the other.

Non-essential accessories that are still nice to have include:
-Water bottle and holder so you can have a drink without stopping.
-Cycle computer if you're interested in speed and other data.
-Rear cargo rack so you can carry your things on the bike rather than on your back
-A kickstand to increase your parking options and make loading the bike easier.

Skip the fenders; they're overpriced and disappointing. You're still going to get wet if you ride more than a mile with them.

^^^
If you're riding in a rainstorm, yes you will get wet with or without fenders. But if you're just riding on wet roads post-rain, or in mist or light drizzle, they absolutely keep you dry. I rather like being able to stay bone dry while riding on wet roads; I also enjoy not being sprayed constantly in the face by my front tire when riding in the rain.

That said, they're hardly essential. If you want to keep it cheap or have problems fitting them on your bike, just carry dry clothes with you, or keep some at work for wet days.

While I will err on the side of your PNW knowledge, I will say that fenders make my life extremely nice. I expect to get wet, but here in the Red Clay South the roads will make you FILTHY. I also can handle the minor rain wetness from falling precipitation, but getting to work with completely soaked through pants and shoes is just not fun. Also, before I got my completely city-ready bike, my commuter was fenderless, and it flat out sucked. Rocks, sand, water all flew in to my face. Its not all about the wet.

Purchase your accessories at the same time as the bike. The margin on accessories is much higher than on bikes so you should negotiate for free extra bits rather than cash off the bike.
Fenders will keep you clean rather than dry, esp if they are full-length bolt-on style. Road runoff is a mixture of oil, diesel, rubber dust and dog doo. I dont want this sprayed all over the back of my waterproof jacket.

Everyone is aware of helmets and safety, whichever side of the debate you stand. You should also consider cycling mitts/gloves as a safety issue. If you come off you are far more lightly to injure your hands than your head. With gloves you just get a few bruises and can ride to work. Without gloves you skin your hands, visit ER and take about 2-4 weeks to recover use of your hand.

Spare inner tubes are useful. I carry one, keep one at work and one at home. You can repair them at your leisure rather than by the roadside.
The most useful item of clothing is a lightweight, bright, cheap windproof (non-waterproof) with some reflective trim.

I will choose to disagree with you my favorite time to ride is during rain and beaing wet dose not bother me at all. Plus I don't ever get spray in the face by my tires or I just don't pay attention to it. Are fenders nice thing to have? Sure, bu when I ride 95% of the time it is not raining and if it dose I am already wet and don't really care. It is kind of the same with paniers are they nice sure, but my backpack is just as nice and more practical than most paniers I ever seen, plus I need a rack for paniers and no additional accessories for my backpack. Essentials to have are only bike, will, and helmet (optional but I rather have one than be a veggy).

Like others have said, a rear rack/panniers will help tremendously if your commutes a few miles+. Day or night as far as I'm concerned, lighting is a must. I have two rear lights, a Planet Bike superflash on blinky and a Cat-eye LD610 on solid. Front light's are great, it does somewhat depend on how often you ride at night. Either way, Don't skimp. Do get good lights regardless, But not all daytime situations call for super powerful lights. I commute everyday in the dark and I have a set of Nite Rider X2 Duals, 300 lumens and they do the job for me. I think these would be more than adequate. There are cheaper options(cygolite, among others) I also have a cheap Energizer headlamp strapped to my helmet. I cannot speak for riding in the rain. I always carry two tubes and a patch kit and basic tools(allens/mini phillips/tire levers, etc..) If I get a flat I just swap the tube and go on my way/patch the flat tube later. A pump is mandatory. Some swear by CO2, But I haven't tried that yet so I can't give an opinion. Also wear hi viz(construction vest etc..) clothing. Do everything you can to make sure the cars see you.

If you'll be riding after twilight lights are essential. Even if you can see just fine a light makes it easier for drivers to see you. A cheap hi-vis vest wouldn't hurt, again just to make sure you're seen.

If you're going to have to leave the bike out of your sight a lock is essential. Don't cheap out on the lock. My LBS told me of one of their customers who bought a top-end mountain bike and then spent the equivalent of about $20 on a lock for it (against their advice). Not surprisingly within a couple of weeks he was back - his bike had been stolen.

If you can get to and from work using some other mode of transport (be it train, bus or walking, or a spouse who can come and get you in the car) you can get away without things like a spare tube or puncture repair kit. Obviously you don't want to be walking 8 miles home in the dark and rain if you get a flat and can't fix it. Replacing a tube is easy enough to do but like others have said learn to do it before you've got a flat in the dark and rain.

Multi-tools are great but only if you know what to do with them. There's no benefit at all in having a bunch of assorted screwdrivers and wrenches if you break something in a remote area and don't know what to do with your collection of wrenches.

If you're not carrying a lot of stuff you can probably get away without panniers. A backpack isn't a lot of fun - it will makes your back wet through with sweat. If you're not carrying much you might get away with a small bag or a saddle bag. For slightly larger loads something like Topeak's DynaPack might work - it looks almost like a bubble that attaches to the seat post. It looks neat in principle although I've never used one so can't say if they are any good in practise. If you're routinely carrying lunch, laptop, change of clothes, washing gear including towel for showering etc then obviously you'll need something bigger.

Personally I wouldn't cycle without a helmet, but others would disagree on that.

Bike, lock, and lights. I put lights above a helmet because "what if it gets dark?" seems to stop people a lot more than "what if I hit my head?" (and honestly, I consider that sensible... the worst bike accidents tend to be at night). The lock's price should go up with how difficult it would be for you to replace your bike. If you've got a $1000 bike and a minimum wage job... you want a damn good lock. If you've got a $50k/year job and no dependents and a vintage 3 speed that you saved from going to the dump, maybe the lock isn't so important. Same idea goes for lights. If you expect to ride a lot at night, you want good ones. If you expect they're for emergency use only, you can skimp.

In some states, gas stations are required to offer free air. In other states, it's customary. And then there's where I grew up, where it's 25-50 cents a pop... Scout around and find out which applies to you. Your tires will need to be checked for air at least once a week, and the higher pressure the tire the more often you should check. You can get by for quite a while off free air at gas stations. It is a good idea to have a floor pump and a portable pump, but that can easily run $70-80 to get both. Scout before you skimp tho, since tires that are low on pressure tend to get lots of flats. If you determine that you must get a pump right away, you can usually get a cheapie floor pump at Walmart or Target. The portable pump tends to be the really pricey one.

If you don't know how to change a flat, figure out how many bike shops are on your commute route. I remain firmly convinced that my bike knows when I've traveled out of comfortable walking distance to a bike shop. It is a very fretful bike and will have flats whenever we go too far from a bike shop. You can counter this natural bikeish tendency by learning to fix flats and carrying the tools, but flats are a part of biking. Again, this is an area where you can skimp, but scout so you have a plan first. I've often joked that my flat kit is my bus pass... but as I've gotten to be a stronger rider, I have gotten to the point where I might go out on a 20-30 mile loop of errands. Not all of those miles will be within reasonable distance of a bus stop.

As far as safety gear... you'll be the best judge of what you want/need. I'm pretty klutzy, as my collection of scars will show. But so far my worst biking injury was a badly sprained knee. I do wear a helmet, because as a klutz it is prudent. My head isn't very replaceable. But so far, neither head injuries nor scraped hands have been on the menu. If you know you're a klutz, you'll probably end up pretty injury free because you've got that voice in your head asking "and what happens when you screw up?" Think it over, take reasonable precautions, and you'll do fine. (the other voice, the one asking "what if THEY screw up?" you can't do so much about... a lot of the time you're more in danger from other people racing around like maniacs than you are from yourself. the best protection I'm aware of there is to never need to be a maniac. it's very freeing to realize you've got plenty of time and there's no rush and you can use your brakes liberally.)

And what caused my sprained knee? Wet railroad tracks. Certain kinds of falls on a bike are really common, and that's one of the common ones. I was lucky. The worst case scenario for that kind of fall is a spiral fracture of your femur. The stuff everyone is scared of, like getting hit from behind, is rare. Most bike crashes and falls are super predictable, and you can learn how to prevent them. That is 100% free, and absolutely worth doing. (naturally, most of my bike falls have been totally not common stuff... but the one common one was awfully dramatic.)

<-- Also lives in the PNW (Wet Coast of Canadaland). Get the fenders. Jus' sayin'...

Unless you have an expensive bike, you'll probably spend more on accessories than the initial bike purchase. Budget at least $500 to start. You'll need a helmet, water bottle/cage, CO2 inflator, spare tube, tire levers and saddle bag so you can quickly fix a flat and get underway. Fenders, gloves, lights, rack, trunk bag, and cycling shorts would be my next purchases. Then clipless pedals and shoes. The list goes on and on.