Get yourself a book of scales.

The scale that fits the Dm7 chord is the dorian which is simply the C scale going from Re to Re rather than from Do to Do. If you know all of your major scales the scale starting on the second step of the scale and using all the same notes will fit the minor 7th chords. For example scale for Am7 is the notes of the G scale staring on A. The scale for F#m7 is the notes of the E scale starting on F#. The scale for Bbm7 is the notes of the Ab scale starting on Bb. Hope this helps. John