As was mentioned before, most races allow anyone to participate that has at least basic understanding of sailing rules of the road (starbord/port, windward/leeward, etc.). Local "beer can" races are a hoot, as there are a handfull of various boats and plenty of room. The get togethers after the races are great sources of info. And the best part is you probably don't have to trailer your boat very far or spend an entire weekend at an event. Most local races are short, on a weeknight or weekend afternoon, and the racing isn't too serious.
And the other advice of hanging back a bit at crowded starts and rounding a mark wider than a tangled group of boats can be sage advice even for the seasoned racer!
The primary rule is that it is to try and avoid a collision, even if you have "right of way".
A quick read of the sailing rules of the road, and general racing orientation (course types, starting sequences, etc.) will help, and in the local race stuff, your competitors will be more than happy to offer advice and help!
So get out there and have some fun!
J