My ex-girlfriend just had a baby and says it's mine. I don't want anything to do with her or the baby. If I sign a waiver of paternal rights, what does that do?

Normally nothing unless the child's mother and the judge will agree.

But it merely means you can not have a say about the child . But then if you just don't, then it is the same thing, normally the mother wants it, to keep you away from the child.

It does not stop child support, you will still have to pay that.

It does nothing really. All you really can do is not sign the birth certificate and go to court to request a DNA test. Then if the results come back to the baby being yours then you will be ordered to pay support and visitation can be set up. If you do not wish to have visitation that is your choice but it won't get you out of anything.