Various long-distance trains arrive and depart at Frankfurt Central Station (Hauptbahnhof). From there, you may take an arbitrary S-Bahn heading direction Konstablerwache and exit at station Hauptwache.
Continue your trip on U-Bahn U8 and exit at station Uni Campus Riedberg after a ride of 20 minutes. Alternatively, at station Hauptwache you may take U-Bahn U3 and exit at station Niederursel, then either take a 10 minutes walk uphill, or change to U-Bahn U8 or U9 and exit at station Uni Campus Riedberg.
During daytime the trains of U8, U3, and U9 run four times per hour each, and inner city S-Bahn connections are even more frequent.
Timetables and Connection Details
Source of information: Deutsche Bahn, the national transport provider.