The city of Frankfurt runs a highly efficient public transportation system. There are urban railways (S-Bahn, mainly above the ground), sub-ways (U-Bahn, mainly underground), trams (Straßenbahn) and buses by the minute throughout the whole Frankfurt area and the surrounding municipalities.
Link to the Rhein-Main-Verkehrsbund (RMV) for all public transportation options and connections inside and outside Frankfurt
Many people prefer to take a bicycle to work. This is certainly the healthiest way to move around Frankfurt and you will find that the city is rather small in size. Buying a second hand bicycle might prove to be a good choice for you.