Generally, to be eligible for expungement, you:
1. Must be a first offender
2. A certain amount of time must pass between conviction and requesting the expungement of that conviction (for misdemeanors, that time period is one year)
3. Have no pending criminal charges at the time of the request
4. Have been "rehabilitated".
5. There are some crimes, like driving under the influence, that you can never expunge.