The first thing you need to do is determine what type of bike you want. For most people cycling to college, a road or commuter bike will be the best choice. Look for a light and durable frame (aluminum or lugged steel, no need to pay more for fancy carbon fiber frames unless you have lots of money to burn - they're lighter, but they might not handle potholes as well and cost a lot more to repair).
Make sure to get a bike that either comes with fenders, or is compatible with them. That's extremely important. Also make sure that the bike you pick allows you to carry all the things you will need for school. You can always use a backpack, but many commuter bikes will also have racks/panniers that can make your life easier.
Road bikes can be faster - and at first that might seem like a big benefit - but remember that the seating position can be less comfortable than on a commuter bike (which has a more upright seating position), and that if there are lots of traffic and red lights on your route, the extra speed won't matter too much. Better to be comfortable, because if you don't enjoy riding, chances are you'll just give up.
If your college is in a flat area, you can save some money by getting a single-speed or fixed gear bike. But if you are in a hilly area, probably better to go for a multi-speed bike unless you are very fit and want the challenge.
Finally, make sure you get a good helmet and wear it.
For more on about a green college experience, check out Back to School Green.