Well, you haven't given us much to go on. What kind of riding will you be doing? Around town, on campus, mostly on roads? Gravel roads and light trails at the park? Or more serious trail riding?
I also don't know your budget, so I'll assume it's low and you're new to the game. Go to a local bike store (aka LBS) and try out a few bikes. Don't be scared to ask questions of the guys there, and don't be afraid to ask for a test ride. They will help you get a bike that fits, and give you other good suggestions. The main brands you will likely find are Trek, Giant, Cannondale and Specialized, although there are dozens of others. I've linked to the low-end of each brand to give you an idea on price. They are all roughly the same in specs and quality, although the Specialized is the Bike Forum approved entry level bike for beginners.
After you go to the LBS and get a feel for the bikes and their specs and what size you'll need, you could also go to Craigslist to see if you can find one there (which you often can). Get as new a bike as you can off of Craigslist, because people usually way overprice older bikes, and often don't take good care of them over time.