Decide whether you are going to form a sole proprietorship, partnership, LLC, corporation, non-profit or cooperative. SBA's guide on determining business structure is a good resource.

If you will be starting a corporation (stock or nonprofit), a limited liability company, or a partnership (limited or limited liability, you must file with the California Secretary of State's Office.  It is optional for general partnerships to file with the Secretary of State and sole proprietorships are not required to file.

California Secretary of State
California Business Portal
1500 11th Street
Sacramento, CA 95814
Phone: 916-657-5448

Estimated Cost:  Varies. See Fee Schedule.