Watch the Mixolydian Scale online guitar lesson by Bruce Arnold from Jazz Guitar for Beginners
The Mixolydian Scale is used over a dominant 7th or a sus4 chord or any Mixolydian or 7sus4 tonal center.
A C Mixolydian scale C,D,E,F,G,A,Bb,C is the same scale as a F Major Scale. Again you want to resist the temptation to play over a C7 chord tonal center with a F Major Scale.
Remember to think the notes or the degrees when learning the C Mixolydian scale and use a jam track for C7 as a backing track so you hear the scale in the right relationship. You should of course know this scale in all keys.