You can file your own motion. The forms are available online, but I usually suggest visiting the clerk's office at the court where you convicted to request their packet, which contains the forms, any additional local forms, and instructions.
Before your start, you should be aware of the limitations of a so-called "expungement."
Under Penal Code §1203.4, a person who is granted probation can apply to have the case dismissed after probation is complete. This is sometimes called an expungement, but it doesn't really expunge anything or seal your record. The conviction is still a part of the court's public records, which will also show the case was dismissed.
You still must disclose the conviction when applying to be a police officer or for certain other jobs, or if asked on an application for a state license or to run for public office. It will not remove any restrictions on your right to own a firearm.
BEWARE of law firms that promise to "clear your record" and charge thousands of dollars. The process of filing a Penal Code §1203.4 petition is pretty simple, and you can do it yourself with forms available from the court clerk's office, especially if you completed probation without any problems. If you had a probation violation along the way, you may want an attorney's help, but the guys who charge thousands to file simple paperwork will probably overcharge you for that, too.
California Labor Code §432.7 says employers can't ask about any arrest that didn't result in a conviction, inquire about it from other sources or use it in a hiring decision.
Some attorneys interpret this Labor Code section to mean you don't have to disclose a conviction that was dismissed under Penal Code §1203.4, but I usually advise clients to disclose it, with an explanation that the conviction was subsequently dismissed.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this site, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.
The form is available on line or at the clerk's office however the form is not self-explanatory. There are choices you have to make which are not readily clear to most folks. If you want to proceed with this and your were on probation (required for expungement of a felony) I suggest you call your probation office evn though the case is old. They normally do it for you for free.
You may be eligible to file a Petition for Dismissal (an Expungement). The form seems simple, but can be somewhat complex since you need to know all of the specific details of your conviction.
Feel free to contact me via www.easyexpunge.com and I will see if I can help you.