The Dorian scale in the Key of G major starts with the note A and then you would play through the rest of the tones in the scale ie: A, B, C, D, E, F#, G, A. The Dorian mode also has a step pattern where the 3rd and 7th tones of the major scale (G in this case) are flattened so you get (1, 2, b3, 4, 5, 6, b7) or (T = Tone, S = Semi-Tone: T, S, T, T, T, S, T) so referring to the scale above if you follow the step pattern you'll find that it works out. The modes really get interesting when you throw some modal chord progressions behind them....but lets just start with the basic for now. Make sense?

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