What state? Some states after so much time your civil rights are restored. Some state no. Some crimes, no.
What kind of a crime?
Some states you would petition the court. Some (Wyoming) you have to try for a Pardon through the governor or a Restoration of Rights.
It could be simple, it may already be that you can vote, or it could be a nightmare and may have to go as far as Federal court.
What state?