All depends on where you are located. Here in the northeast, we have USCRA and some AHRMA races. There are a bunch of different organizations throughout the country.
BUT before you can race you must take a road racing school. Penguin Road Race school is the one i went to. You can take Keith Code's California Superbike school also, and there are others, but i cant think of them right now.
I'm going racing next year also, and vintage racing seems to be a lot more fun, wringing the bike out to max instead of going 7/10ths on a modern sport bike.