Here in the midwest, at the amatuer level, its purchase a ama card. $39 good for one year, and you have to have a District Card. That is a card that records your season point standing in that Card's District. Here they are $20 and also good for the season. Then they get you again at the gate, then again to race each event. 1st race costs the most.
All of the races that I attend, have the appropriate paperwork on hand for a new guy to show-up and then race. With the Ama Card purchase, you will receive "American Motorcyclist " magazine, as one of the few benefits, which gives you event schedules for all AMA sanctioned events. The district card here, not sure if everywhere, sends us schedules for our Districts (area).
I would say, find the race to bust your berry on, and contact that Club. They are usually excited about their upcoming event and full of information about the race and the "rules." With quotes, because different clubs "interpret" the rules differently. Its not a hassle at all, though, like I may be making it sound like. You just don't want to dedicate a weekend to racing and find that your bike needs a killswitch or something. The biggest work is getting the bike ready and every spare part in the truck. Hillclimbing is a blast too...