my child has reached 18 and grad. how do i stop support payments now from coming out of my check?

Have you seen a copy of the order, was there a stop date ?

You file in court to get the order withdrawn

Support is automatically terminated unless you are in NY

Support SHOULD be automatically terminated (unless there are arrears being collected). But practically Support collection agencies aren't all that conscientious. However, this may be the fault of your employer. They may not have checked that the order should have expired.

So the first thing I would do, is go to payroll tomorrow with a copy of your support order and your child's birth certificate. Ask them to remove the garnishment. If they refuse then you need to go to the agency collecting the support and ask them to release the garnishment. If they won't, then you have to go to court to get a court order to cease collections.