2 ways to go about it,dpending on your state.
1.Go file in family court.
2.Go to your local child support agency.
Usually child support if you file will bring you into court to decide on what should be getting paid based off information provided.
You or your ex may need to pay these court fees but like i said each state varies so call the clerk of courts and ask to get it started.
Also if you feel it is too much to do on your own call a family law attorney.
Good luck ~
at 2:56 AM on Feb. 5, 2009