How to use this circle:

     If the key is "C" (major) the letter left* left of the "C" will indicate the subdominant (F) and the one on the right* the dominant (G). So you have to use the chords C, F and G. If there are minor chords in the song, they will be Am, Dm and Em (Usually it would be Em7, but that means going deeper). (Right* means here clockwise, Left* means the other way around).

If the key is Bbm, all* possible chords were: Bbm, Ebm, Fm, Db, Gb, Ab. (*If the song really sticks to the standard scheme, some songs have additional chords which don't fit to this rule.)

The parallel minor scale to C is Am, the parallel major scale to Fm is Ab.