You must have totally completed your sentence. You must present a motion to the court requesting it, accompanied by your reason(s) for wanting to do so. A judge will review your case, and your motion, and make a decision either granting or denying your request. An expungement will only remove the record of your offense from the "public" portion of your record. It is not a "pardon," as far as the legal system is concerned you will always remain 'convicted' of whatever crime you were charged and tried for.