This will be my 1st bike purchase since i was in freshman year of high school.

What accessories should i get along with the bike, not including the helmet.

Oh boy, where to begin? It all depends on what you are planning to do with the bike.

Ride at night? Lights, front and rear.
Live somewhere it rains a lot? Fenders will help keep you and the bike clean(er).
Plan on going anywhere other than just rides around the 'hood? Locks
You might want to consider gloves as well. And don't forget a patch kit and a spare tube. And a pump/CO2 inflation...

I'll let you lob the grenades as far as the mirror and backpack/mess bag/pannier questions

If you want to carry stuff around, you should take a look at a carrying rack or basket. A water bottle cage will probably be helpful, too.
Help us to help you.

Get a bag, or something, to carry a spare tube. Learn to use CO2 - pump(?) if you opt for such - best to learn at home rather than some road next to the Addams Family house. And a pump is good - but don't get a "Mini-Pump" as they don't work well - and are likely to make you rip the valve off your tube. Really! Get one with a separate hose that screws-on to the body of the pump. MUCH better. And toss in a patch-kit. Learn to use at home also.

Now a tool-kit... ... Get a multi-tool. A good one could almost build a bicycle! Many will chime in here - so I'll start:

26 tools - enough to tighten most problems:

https://www.amazon.com/Topeak-Alien-2.../dp/B000FIE4AE

Now you're ready to get back home, or to a phone, in case God hates you.

And get good lights - the Search should help you find lots of rock-throwing apes making suggestions on this subject.

That nicely explains my Sunday morning commute then.

I think it's important to have a rear rack with a trunk bag to carry your wallet, keys (it's uncomfortable riding with these things in your pocket), cell phone, any cable lock you might have, spare tire, mini pump, patch kit, multi-tool... okay, so I don't like to leave the house unprepared

You will need other things if you are planning to go above and beyond normal riding. If you are planning on riding great distances, do yourself a favor and get bar ends. Also, a bike computer is a fairly cheap toy that can make riding more fun.

Water bottle cage and bottle, some shops throw this on the new bikes. Gloves, depending on the bike these may make a difference in riding comfort, they will help with grip in any case. A seat bag to hold a spare tube, wallet, phone, patch kit, and energy bar. That should cover any situation for a few months. BIKE SHORTS, trust me. spend the $ for a good pair, 6 or 8 panel. How are you planning to ride? Think clipless pedals and shoes, or at least cycling shoes. The stiff sole help keep your feet comfy by spreading out the pressure on the bottom of your feet. Most shops will give you x% off purchases when you by the bike, or for a period of time after, use it.

what are the benefits of clipless pedals?

Like toe clips and straps, only better.
Let the games begin.

In typical interweb fashion, I'll explain even though I have never used them.

Take a look at how your legs move while spinning the cranks. One leg is doing all of the work while it is in the down stroke, while the other leg is very limited (muscle exertion) in the up stroke. By fixing your foot in place to the pedal, you will be able to use your muscles on the upstroke as well. Reportedly makes it more efficient.

But like I said, I have no direct experience with them, nor do I plan on using them, until I get a recumbent trike.

When it comes to loading your bike with accessories, it pays to think about integration. Will all the bits play nice. Is there anything you can do to speed up the lockup/unlocking procedure.
Some racks have light brackets at the rear so you have a light tucked below the rack, not obstructed by large loads on the rack top.
When you stop, do you want to remove all your accessories or do you want them fixed.

My all-year, all-weather commuter has fenders F/R (SKS chromoplastic), rear rack, front dynohub with the lamp on the fork crown and rear lamp on the seatpost. The pump is in the seatpost. Usual luggage is a single side pannier. U-lock goes on top of the rack, held with a bungie.
I upgraded the bars for comfort (On-One Mry) and added Ergon grips.
I removed my toe clips for short-range journeys.

No kickstand, mirrors, bar-ends.

What accessories should you get? Depends entirely on your needs.

Lots of folks here are recommending stuff you don't really need.

At minimum, you'll want a water bottle cage and bottle - a good LBS throws this stuff in for free.

You'll want a tire patch kit and a tire lever or two. A good mini pump goes with this.

Multi-tools aren't really necessary except for really long rides. I always carry a multi tool, but the only tool I've used from it on the road is the bottle opener.

If you ride at night, get some lights.

If you need to carry stuff, get a rack and a rack trunk, or panniers, or something. If its not much, a backpack will work fine for many people (not me, I hate backpacks while riding).

Fenders are nice if you plan to ride in the rain.

Everything else is really on an as-needed or wanted basis. You can get by with very little. There's lots of accessories for bicycles out there.

Clipless pedals, toe clips, all that stuff is not needed at all. If you want maximum performance, sure, clipless pedals might be a nice addition.