My understanding of your question is that you are looking to rent a room in someone else's house and be their roommate. If that is correct, Eric's advice to go to craigslist is right. Their website is http://www.craigslist.com -look under the Housing categoty for rooms/shared or sublets/temporary. If that is not your question if you clarify I'm sure we'd all be happy to help.
Lance King/Managing Broker
Jeff Woo, Esq.
Sedgwick, Detert, Moran & Arnold LLP
Complexx Rental Property Group
It's really the best place to look, and if you're new to the city, again, I'd be happy to assist you.
Just visit craigslist.org for more info. In fact here is the exact link http://sfbay.craigslist.org/sub/ for the craigslist sublets and temporary housing.
There's also a section where you can list what you're looking for called "housing wanted."
Good luck and again, let me know if I can help in any way.
Eric M. Abrams
CA Real Estate Broker