To learn the dominant seventh chords, all you do is lower the last note of the major seventh chord. For example:
The C major seventh chord is (C, E, G, A), so the C dominant seventh chord is (C, E, G, Ab). The last note A is lowered a half step to Ab.
The Db major seventh chord is (Db, F, Ab, C), so the D flat dominant seventh chord is (Db, F, Ab, Cb). The last note C is lowered a half step to Cb.