i would race the beginners class if i were you. cyclocross races are a lot harder (depending on how hard you push yourself) than they appear. if it's just master's and not master B's and master A's...definitely race beginners. them masters are fast! master B's are typically as fast or faster than the majority of the C racers (minus the sandbaggers).
i would suggest going to the clinic if you've never raced before. it should give you a good feel for the course and let you see what your going to have to go thru before your eyesight goes all blurry and you can't breathe.
if you haven't raced before, just go out there and have a good time. start towards the back, let the people that are coming up from behind pass you, and just enjoy the pain. it's temporary.