Ask your attorney to file a petition and order for expungement, Judicial Counsel Form CR-180. You can do this yourself, actually, although your attorney will probably have a better shot at succeeding. Here's the link: http://www.courtinfo.ca.gov/forms/fillable/cr180.pdf.
There's a $60 fee, payable to the clerk of the applicable court. If you do it yourself, be sure to serve the prosecutor, and to add a certificate of service to the form you file with the court.