First and foremost, check Craigslist. It's an invaluable resource for many areas in California.

Here is a page that will give you basic guidelines for finding a house if you are in college or just otherwise searching:

•Guide to Bay Area Housing

There is a very nifty site called ApartmentGuide.com which can help you find an apartment that suits your needs. The link below is a specific search for SF Bay Area apartments, but if you go directly to ApartmentGuide.com you can search in a broader range.

•SF Bay Area Apartment Rental Finder

Addition provided by cutemer02.
More Apartment/House Finder Links:
http://www.springstreet.com
http://www.rent.com
http://www.realtor.com

Find A California Realtor with Realtor.com