Find a local race that you want to go to. Then show up early! Like 7:00 or before
I would go straight to sign up (if AMA, have your card with you and district card) they can also give you some insight as to the days time frame.
There usually is a referee at AMA events, he can give you a hand on the time schedule of the days events. Then just start asking around, usually people are pretty friendly at mx races. Most practices start at about 8:00, so be suited up and bike ready. If its your first race, you probably would like to find a non points race for your area.
Aztalan Cycle Club VP
Road Toad & others