To learn the minor chords, all you do is lower the middle note of the major chord a half step. For example:
The C major chord is (C, E, G), so the C minor chord is (C, Eb, G). The middle note E is lowered a half step to Eb.
The G major chord is (G, B, D), so the G minor chord is (G, Bb, D). The middle note B is lowered a half step to Bb.