Find out from your local district office; as Dragwyr said, it’s often the HR person. There is usually an application, sometimes also fingerprinting and background check. Not sure how they do it in CA. Then the substitute registry will call you when they need you.
A few hints about being a successful sub (I’ve been both the sub and the teacher, so don’t take it personally if some of these tips seem elementary): Register in more than one district, if you can. Take jobs when offered, every time you get called. Show up on time. Dress professionally. Follow the lesson plans very carefully (once, I asked a sub to show a movie and give out a hand out. He showed the movie but the stack of handouts just sat there :smack: ) Leave the teacher a detailed note about how the day went, who was absent, etc. Doesn’t hurt to say “thanks for having me, it was great working for you, and please request me the next time you’re going to be out.” It works. Don’t let the kids walk all over you. Doesn’t hurt to know the discipline procedures for the school, as well as the general way things work.
Good luck, man. Quality subs are worth their weight in gold, so if you can pull it off, you’ll be booked solid in no time.