Call 1-888-208-4485 (Division of Child Support Services, Customer Service) to find the address of the location nearest you. You must visit the office to file your case.
Depending on the individuals monetary situation, I would recommend finding a lawyer specializing in divorce and family disputes. A civil suit is the best way to collect child support.
The fastest method to get help anywhere is to let your "fingers do the walking in the yellow pages". The court would probably provide the next best help. A modern method is to enlist the services of a collection agency.
