Catch the S-Bahn in to town. You want to head to the Zeil as Fanatic25 said. The shopping there is quite good IMO, lots of department stores amongst other shops too.
Get the S-Bahn to Hauptwache (no change needed), which is at the upper end of the Zeil, you can then walk down towards Konstablerwache and see all the shops. If you're done when you get there, you can get the train back to FRA from Konstablerwache (again, no change needed).