The S-Bahn brings you from FRAPORT to the city. Entrance at basement of FRA.
Hauptwache (Place) is a good central point to start. The nices shops are at GOETHE STRASSE and (parallel) BOCKENHEIMER.. CALLED FRESSGASS (STREET WHERE YOU EAT).
Also visit ROEMER. There is a christmas market. It's near Main River. On the other side is Sachsenhausen with APPELWOI restaurants.