Melvin Said:

What should I do to become a social worker in California?

We Answered:

You sound very intelligent. I don't know the answer, but I bet you would already be qualified to some extent. I would start by looking for jobs on http://socialservice.com/search.cfm which seems to allow you to put in the state (California), position you want, and the degree you have already earned in Japan.
Your english sounds fine to me!
You could also try searching at http://www.socialwork.com/

Good luck with your green card & job and everything!!!