Whatever you do, don't make the mistake of going out and grabbing a Wal-Mart special just because they're cheaper. The bike will almost surely not fit you well, won't shift or brake properly--at least not for long, and you'll be so unhappy and uncomfortable with the the bike that you might think you just don't like cycling, when it's really the bike that's at fault, not the sport.

A decent bike is a good investment, so bite the bullet and make it--and, oh yeah, have fun.