The Dorian scale is an organization of pitches encompassing the Dorian Mode. If you are looking at the piano, it would be all the white notes from D to D. The pattern of tones are as follows:
Tone, semitone, tone, tone, tone, semitone, tone
The order is in reverse for the descending scale.
Listen to this Dorian scale and then repeat the recording to sing along. Notice where the tones and semitones fall.
T = Tone (Whole Tone)
ST = Semitone