Any good brand rubbing compound, and there is actually one for dark, black vehicles, then "finish" it with a good finishing glaze, they make one (3M I think) for black vehicles, I use it. Comes in a big squeeze bottle.
As for the rubbing compound get a SOFT (non terricloth, since terricloth scratches) rag or towl, apply the rubbing compound to the towl and start rubbing HARD in tight circles, and back and forth. You will feel the rag getting hot from friction (good thing) do that for a few minutes or until you see the scratch dissapear. Let the compound dry a few minutes then wipe it off with a clean part of your towel. You won't need a lot of compound, say enough to just cover the scratch your working on. Then the final step is just hit it with some finishing glaze!