Look up lawyers in your area and make lots of calls. I was in your boat, never married but have a custody agreement. Find somebody who will guide you through the process and let them know you don't really know how this all works, so you need them to be the expert and explain everything to you.
Google "Family Law" and you should get results for your area. Also try "Family Law FAQ" and start looking over the kinds of questions you want to be asking attorneys when you talk to them.