If you want a buzzed look on the sides/back and slightly longer than buzzed on the top, ask for a #2 on the sides and a #4 on top. For short, but not buzzed-looking, ask for a #4 on the sides/back and a #6 or #8 (#8 is 1 inch of hair) on the top.
A #2 clipper is 1/4" of hair, and you will be able to see your scalp through it. #4 is 1/2" and your scalp should be hidden.
Hope that helps!