Determine whether you were convicted of a felony or a misdemeanor
Felonies can't be expunged in California, but you may be able to have a "wobbler" offense reduced from a felony to a misdemeanor, which may be then be possible to expunge.
Contact the court where your conviction occurred to obtain your records if you don't have them
You'll need the information contained in your court record to begin the expungement process. Information such as the California code section violated, date of offense, date of conviction and sentence will be needed to start the process of clearing your record.
Contact the court clerk to learn about the expungement process in that jurisdiction
Many California jurisdictions have a form you can fill out to petition for expungement, while others will require you to draft a petition or motion. Either way, you'll need to clearly express the reasons why you believe your California conviction should be expunged. These may include indications of rehabilitation such as the number of years that have passed since you've been in trouble with the law, educational programs you've completed, etc. You'll need to demonstrate to the court that you completed all requirements of your sentence, including paying all fines and fees and successfully completing probation.
Submit the petition to the court along with copies of all documentation
California law now requires you to attend a hearing in order to have an expungement petition considered. The court will set a hearing date that you must attend. Be prepared to argue your case for expungement using the same clear and convincing terms you outlined in your expungement petition. You'll typically receive a written ruling from the court within 60 days of your hearing.