Answer:
Every state is different so you will have to check with your particular court system. You will probably need the services of an attorney - it will cost money - you are usually limited to only ONE expungement per lifetime. In the meantime below is a general guideline:
To request the expungement of an offense from your STATE (not Federal) criminal record: You must have been exonerated, acquited, or served the complete term of your sentence - then you must file a petition/motion with the court of the state in which you were convicted (which may not necessarily be your current state of residence) - setting forth valid reason(s) why your request should be granted. A judge of that court will review your petition and the circumstances of your case and issue a ruling either granting or denying the request. AN EXPUNGEMENT IS NOT A PARDON! Expungement only removes the record of your offense from being viewable by the public. Law enforcement, the courts, and government agencies will always have access to your actual 'true' record.