Check out the USCF's page on categorization: http://www.usacycling.org/news/user/story.php?id=580

I'm not totally clear myself, but basically you can move up in category with experience and/or placed finishes in races.

You can also buy a USCF license from that site. Most races will require that you show your license to be placed into the appropriate category's field. Some races will have a citizen's field for people without a license.