You can register to vote up to 15 days before a election day which mean the deadline for registration this year is October 22, 2012.

To register, you need to complete a voter registration form.  You can obtain a voter registration form from the Registrar's office or many public buildings, such as libraries, the DMV, Post Offices or the Eastvale City Clerk.

Riverside County Registrar: www.voteinfo.net
2724 Gateway Drive
Riverside, CA 92507
(951) 486-7200
Toll Free: (800) 773-VOTE (8683)