You will need to be more specific on what printer model you are referring to.
If it is a printer (typically an HP OfficeJet) that has the fax component, then you would need an VoIP (voice over IP) service (similar to MagicJack) that would provide a converter that would convert analog signals (fax) to digital and use the internet. But that would mean you need to pay for a phone service and if you are looking to do one fax, then that would not be feasible. If you have phone service as well as internet with AT&T, then you can connect the printer that way. You will need to contact AT&T for any potential setup.
If your HP printer has no fax capability, then it cannot be done.