The Mixolydian scale is an organization of pitches encompassing the Mixolydian Mode. If you are looking at the piano, it would be all the white notes from G to G. The pattern of tones are as follows:

Tone, tone, semitone, tone, tone, semitone, tone 

The order is in reverse for the descending scale. 

Listen to this Mixolydian scale and then repeat the recording to sing along. Notice where the tones and semitones fall.

T = Tone (Whole Tone)

ST = Semitone