You can register online on the Secretary of State's website.
You can also fill out the short Registration by Mail form. Sign the form and mail it. It's pre-addressed, and no stamp is required. Voter registration forms are available at the Registrar of Voters Office located in the Alameda County Courthouse:
1225 Fallon Street, Room G-1
Oakland, CA 94612
Forms are also available at all offices of the Department of Motor Vehicles, all city clerks offices, public libraries, and post offices. You may also call the Registrar of Voters at (510) 267-8683 or the Secretary of State at (800) 345-VOTE and request a registration form to be mailed to you.