Watch the Dorian Scale online guitar lesson by Bruce Arnold from Jazz Guitar for Beginners
The Dorian scale is used over any Minor or sus4 tonal center. A C Dorian scale C,D,Eb,F,G,A,Bb,C is the same scale as a Bb Major Scale.
Resist the temptation to play over a C-7 chord tonal center with a Bb Major Scale. You want to avoid this because although it will work you are not thinking of the scale the same way that you are hearing the music. Making this mistake over time will severely limit your success with music.
Remember to think the notes or the degrees when learning the C Dorian scale and use a jam track for C-7 as a backing track so you hear the scale in the right relationship. You should of course know this scale in all keys.