just got out of work and went to start the TL. Put the key in and all I hear is one click.

Headlights, radio, all lights still work.

I tried jumping it with a buddies car but still not starting. It sounds like the click is coming from the starter. I tried banging on it but nothin Still !

Would this be a dead battery? If so how do I test for that ? Or do I need a starter replacement?

 

Probably Starter.
If the lights stay bright and only a click is heard, starter. At times, whacking on the starter is not always a surefire way of getting it to work. Occasionally one can turn the key numerous times and it might work,
but no guarantee.

Daughter's Hemi Grand, starter went and no go, hitting or jumping the starter wouldn't operate, installed new one, fine.
Last Tuesday, other daughter's fiance same problem with his GTI. Stopped at a convenience store and that was it. Flat bedded to garage, starter replaced, go to go.

 

More than likely the starter as Turbo said