If you are a California resident, you can register to vote online at http://registertovote.ca.gov/. To vote in an election, you must register to vote at least 15 calendar days in advance.
If you do not have a California ID you can register to vote by filling out the online form and the state elections bureau will mail you a form to sign.
If you are not registered to vote by mail, you can look up your polling place on the alameda county website.