For beginners, see if the Socal site has any clinics and tried go to them, if possible. You will pick some tips that will make the first race much less stressful. Also, if you have a mtn bike, use it for the first couple of races to see if you even like cross; just remember to take off the bottle cages and bar ends. If not, an old road bike might work if you can get 27mm cross tires to fit into the rear triangle. For dusty, dry courses, this setup will work but for mud, the fork and the chainstays will get loaded with mud.
Finally, see if you can find some local rides so that you get some tips on how to dismount and remount. Remounting will be the toughest skill to learn.