I just found out that a girl I went to high school with back in 1997, is claiming that I am the father of her 12 or 13yr old son. I wanted to know if I would have to pay back childsupport.

In what manner is she claiming you are the father? Has she filed for child support, filed for a determination of paternity?

In "most instances" child support goes back to the date the custodial parent files for support, not to the date of birth.