Your first resource should be American Whitewater:
American Whitewater - AW Homepage
have good run descriptions.
You should do a search and see if there is any whitewater clubs in your area and join up.
Try and hook up with other paddlers that are more experienced and learn all they are willing to teach...if there is anywhere in your area to do so, take a course.
Try and find some used gear that is not too old, but not too expensive (beginners tend to lose gear)...
and good luck! Paddling is unbeliveably fun and fullfilling, but can be dangerous...
Welcome to the sport.