Based on my experience, So Cal is one of the more competitive regions. Decent fitness is a bare minimum. If you don't have any experience riding in a fast and close-quarters pack, I suggest you get some before the season starts. Find out what the local hammer ride is in your area and do it. Hang out near the back until you feel comfortable enough (and strong enough) to move around. Start reading up on specific training routines. Joel Friel's Cyclist's Training Bible is a good start.

Finally, remember that many people get dropped on their first race. It's just something you can't prepare for without just jumping in for the experience. The biggest downer of bicycle racing is that once you're dropped from the pack, you're pretty much done.