To learn the minor seventh chords, all you do is lower the second and fourth notes of the major seventh chord. For example:
The C major seventh chord is (C, E, G, A), so the C minor seventh chord is (C, Eb, G, Ab). The second note E is lowered a half step to Eb and the fourth note A is lowered a half step to Ab.
The F major seventh chord is (F, A, C, E), so the F minor seventh chord is (F, Ab, C, Eb). The second note A is lowered a half step to Ab and the fourth note E is lowered a half step to Eb.