In California, a new birth certificate should have been issued after the adoption; it will contain the adoptive parents as the parents. The original birth certificate is sealed and is only available on court order.
You should be able to get a copy of the new (after adoption) birth certificate from the county clerk/recorder where your birth or adoption took place. If they do not have a record, contact the State of California Office of Vital Records. Here is a helpful note from the Placer County Clerk/Recorder:
"Adoption records are sealed at the time of adoption. If there is no birth record in the county where you were born, you must contact the California State Office of Vital Records for a copy. For information on how to order a copy from the state, you may call their information line at (916) 445-2684. If you wish to obtain a copy of your original birth certificate, you must obtain a court order. For further information on how to obtain that court order, you should contact your legal advisor."
The State website is at http://www.cdph.ca.gov/certlic/birthdeathmar/Pages/default.a...