The American Motorcyclist Association is the largest and most influential motorcycle organization in the USA. Others that you may have heard of are ABATE, MRF, Motor Maids, etc. Its the AMA, though, that sanctions virtually all amateur and professional on and offroad competition events. To race hillclimb, motocross, dirt track, etc. you first need to join the AMA, and then join the districts you wish to compete in.

Website: http://kingofthehill.org/