There are several ways to go about this ~~
First of all, I would suggest you contact the family courts where you live and inquire about their support enforcement division -- it is becoming more and more common for couties to maintain their own support enforcement offices because of the high rate of non-payment. From there you will be asked to supply them with the court order granting you the support ...they will go about setting up a collection account from there.
If your area does not have this service available you can contact the local office of legal aid or family services -- both have staff that are experienced in either assisting you or directing you to the assistance you need.
Additionally, a lot of states maintain support enforcement agencies - try starting with contacting the family service (welfare) office in your area and ask them for the name/number of the office you need to contact - again you will need to submit the court order and any other paperwork they ask for and they can go about setting up collection.
Speaking of welfare -- a rather roundabout way of getting your money would be to apply for assistance - they will ask if you get c.s - tell them it was ordered but you don't receive it .... they would rather help you get the c.s. from the NCP than to pay you welfare from their budget. I know this may seem underhanded, but if it works it works....
Or, you could always retain a family law attorney and go about filing the papers for enforcement through the courts yourself -- most of all don't let this go any longer --- it took two of you to creat this child and two of you are responsible to support it.
Good luck and God Bless