Not all tracks run AMA sanctioned events, so you need to check with the track you are planning to run at as to whether or not you need AMA & District memberships.
If you do have to join AMA, usually the last 3 numbers of your AMA number is the best choice for your bike number. However, like me, if you are only racing a few events a year, there's nothing at all wrong with selecting a number you happen to like. In the event that you go to a race, and 2 riders in one class have the same number, they will make some accommodations. It's usually a stripe thru the number on one of the bikes. Not a big deal.
The best advice I ever heard for new riders & racers, ride your own race. The first few times you race you will most likely get spanked. Don't worry about that. Just go out there to learn and have fun. At our age, we are in no real danger of going pro, so just have a good time, and don't do anything stupid. Hold your line and try to be predictable, so you don't get someone upset at you. They'll tend to not use you as a berm the next time they lap you..
I rode my first real race at 40. It was at Redbud. It was the most fun I had ever had. I got beat so bad, I think the first moto I finished 2 of the 4 laps. But it was still a lot of fun.