An expungment will retroactively change a conviction to a dismissal. It will not, however, erase the fact that you were arrested and charged with the crime.
Applying for an expungement (called a PC 1203.4 Petition) is a very simple process. The forms are available on most courts' websites and can be completed by simply checking off a couple boxes. Make sure that you have your case number and the exact date of your conviction.
Once you have completed the forms, make at least 3 copies. Take your copies to the court where you were convicted and get them all stamped by the criminal clerk. Leave one copy with the criminal clerk and pay the $150 filing fee.
Take the stamped copies to the DA's office and leave one with them.
You will probably receive some notice in the mail within a month or two telling you that your petition has been granted and your conviction has been expunged. The DA has a right to oppose your expungement and argue against it at a court hearing, but that is unlikely if you successfully completed your probation and you are not currently on probation in any other matters.
It's not clear whether you were actually convicted of a petty theft. If you were not, then there is nothing to expunge. There would be an arrest record, which you would not likely be able to get rid of. If you were convicted, then you can file the expungement yourself. Although, I typically advise clients to hire an attorney so everything is done properly.
Expungement is always worth it. However, filing the documents is never a guarantee that it will be granted, and some prosecutors routinely object. I don't know why. Petty Theft is a very lightweight offense so you shouldn't have a problem.
Did you successfully complete probation? Have the fines been paid? Do you have any pending cases?