Depending on where you are going to school, there may be a bike co-op near you that will have re-conditioned used bikes. Here is the Boston area, 2 leap to mind:
http://www.bikesnotbombs.org/bikeshop
and
http://broadwaybicycleschool.com/
Also, check your campus newspapers and bulletin boards. The end of a semester when students are leaving, and just before vacation periods tend to bring a lot of bikes onto the market as people are looking for money. Police auctions are another source, but they are usually only in the spring. Unless you can ride an exact match, don't try to buy on-line until you know more about your needs and size.