Well, the season just ended.
Starting in late August or September; http://www.socalcross.com will fire up their clinics with races starting in mid to late September most likely.
I'm pretty sure there was a westside training group this year.
Also check the socalcross facebook page, it gets more updates.

I got started by going to one of those clinics.
You can use a MTB to race if you want - or go all in and get a CX bike.