if you have a bike shop that sells used bikes, maybe you'd like to go there. the bikes will cost more than their craigslist counterparts, but (if the shop is good), the bikes will also be tuned up, maybe with a 30- or 60-day warranty, and the staff will help you pick out the right size for your body. they can also help you decide which kind of bike is right for your needs. there's more support there, which makes the bikes more expensive, but it's also probably safer for someone who doesn't know a ton about bikes.

before you get a bike, it's probably a good idea to figure out your route and what you'll need to wear and carry with you. i bought a bike that was expensive and all wrong for my commute. that sucked. the bike was heavy and the gearing wasn't wide enough for me to haul my butt up the hills i had to climb. if it's rainy you probably want to have fenders, so you'll need eyelets or another way to mount fenders, and if you have a lot of crap to carry, you want rack mounts. if it's ever icy for appreciable amounts of time, you might want enough room to mount huge studded tires.

gotta go!@ good luck!