The most obvious choice for c4d to step conversion is to use the original software - CINEMA 4D. You can export from the program toa variety of formats, including fbx, 3ds, obj, Collada, and possibly more.
If you do not have access to CINEMA 4D, you have to use some alternative 3rd party software to convert c4d to step, for example some dedicated conversion solution such as PolyTrans for CINEMA 4D.