If the harp sound isn't "blues" harp, an F harp is needed. That is, unless there's a minor key sound to it. Since I don't know the song or singer, I can't speculate. But either a standard or low-F harp should be able to play in F. If it's a blues sound, what they call "cross harp," you need a Bb harp, so you can home in on the F note (the 5th in the scale). This system gives you some whole-tone-down sounds (EB, not E below the F note, for instance). It's even possible that the song is modal, and you'll need an Eb harp (if it's in Dorian, or the II note).
Good luck with it. I'm just learning cross-harp playing, myself, and it's fun, but not quite so intuitive as straight-harp playing. Modal playing is even odder (think of starting with the first DRAW note as your tonic).