First step is to stop by DHR's office in your area. You'll need to call and make an appointment. Are you in Alabama? If so look here to find info on the procedure, forms to print out & fill out, and find your local office:
https://www.dhr.state.al.us/page.asp?pageid=420Good luck!
PS: DHR can only help with support, not visitation. If he doesn't want to see the kids (girlfriend or whatever) you can't make him. Chalk it up as his loss and be thankful your children aren't around that girlfriend person!