Custody of my son

I surrendered my parental rights for my son to my ex in-laws upon becoming incarcerated. They always said upon my release my son belongs with me -- his mother. Now that I have been released, and I am back in my son's life my son wants to be with me. But I'm not sure how to go about getting custody back. The original custody order was granted in another state. How do I go about getting custody back? The grandparents have no disagreement to the custody of my son being placed back with me.