United States Chess Federation
All tournament players must have current USCF Memberships. Follow this link to join/renew your membership. Do it before tournament day to avoid unnecessary registration delays and confusion.
This site is the clearing house for chess tournaments in Northern California. If my two tournaments are not enough chess for you, check out the tournaments page here for more tournaments.
To play in a tournament, it is useful to know how to record the chess game you are playing. This link provides a pretty good tutorial if you are new to tournament chess.