i signed papers not knowing i would have to pay child support & he gets custody. what should i do

This depends on a few factors. Generally speaking, a person is presumed to understand and read what they signed, so it may be difficult if the papers you signed clearly state you are to pay child support. On the other hand, if there is fraud, or some sort of duress or genuine misunderstanding, you may be able to undo the agreement. A lot depends on the timing as well, so don't wait too long if you want to contest it. You should definitely contact a lawyer as soon as possible.