Any bike will do. A practical, wet-weather utility bike will have fenders and a rear luggage rack, or at least the threaded eyelets on the frame to fit them.
If you fit a rack and get some pannier bags you can carry a lot of shopping on the bike, much more than you can carry.
The most common kind of basic bike is probably a "hybrid" style. Basic MTB is also good.
One upgrade to make is puncture-resistant tyres. The most common is Schwalbe Marathon, the best is Schwalbe Marathon Plus. They are not cheap but cyclists tend to get less leeway from bosses regarding late arrival than do their driving co-workers.

Useful accesories: Lock, lights, helmet, cycling mitts, trouser clips , spare inner tube, pump, tyre levers, wheel wrench, puncture kit, oil/lube.

Now, do you know how to: properly lock up a bike, pump tires, properly clean and oil/lube parts? Things you need to figure out or go to you LBS.

Sad, but true.

Yeah - I knew several co-workers (nationality withheld, but they fall in line with "American" -i.e. convenience obsessed- thinking) who would drive 5-6 blocks from their downtown apartments to the office.

*sigh*

follow this advice. also, look at craigslist for anything used and decent. You can always sell it for the same price if you decide you like bicycle commuting and want to upgrade to something better. Don't worry about panniers. You can just use a backpack to carry your stuff. If back sweat is a problem, then you can invest in a rack and panniers. Just start off basic and upgrade as you see fit. It's all about experimenting.

Yeah, man. 2.5 is simple. I do about 22, round trip. Get something you really like because you will want to start running errands on that bike too!

for 2.5 miles any tires will do. If one go for long distance - one needs have 15-20 min reserve to fix the tire. For 2.5 miles - you will just walk to work and fix your tire at lunch.
$70 wallmart bike will work. any old cruiser, old mountain , old hybrid will do. (i am doing each day 32 miles). If you buying bike that you can not return (not a wallmart one) make sure - it right size frame, and you can adjust saddle height.
Buy new helmet in wallmart, target etc - no more than $15 , if you need helmet - depends on your state laws and your road. Any rain jacket will do. You can use shower cap over helmet and plastic bag inside your shoes for heavy rain.
Keep your tire pump up and oil/clean chain at least once a month.
good luck.

Commute by bike for any reason you want.

This shows my newness. I consider biking more than a mile or so a long way. But we all have to start somewhere. I wanted to add that Craig's List actually has a lot of FREE bikes.Angie's List also reviews local companies. The internet is a wonderful thing, right?

I just started commuting 3 miles one way to work by bicycle a few weeks ago. At 5am in the morning I wouldn't think about walking to work. It's all downhill to work therefore all uphill home so biking only makes sense.
I started with a 20 year old Kona mountain bike outfitted with fenders and slicks. Worked well until I discovered the hubs were wasted, the cassette was toast, the chain was stretched, etc. I have it dissassembled in the garage to repair now. I did get enough rides into to firmly convince myself I am a dedicated bike commuter now, and that made it easier for me to drop the cash on a brand new Specialized Rockhopper Comp. I found that by lengthening my commute by about a mile I can travel by trails 1/2 of the trip. I far prefer mountain biking to road biking and with the new bike it's an absolute blast. Good luck on getting started with your commute by bike! You'll get hooked!

It is a long way for a lot of folks. It was for me. Now, it's not even a warmup. Soon, it'll be the same for you. Keep riding.