The product needed depends on the severity of the scratch.
Very light scratchs & swirls - A good polish (Autoglym Super Resin or Mer etc) - they don't contain silicon like cheaper stuff can. Or Car Plan Colour Magic that polishes but contains a colour pigment that may help to mask a scratch.
Deeper more visible but not through paint - Try a cutting polish that will remove some of the surrounding paint and blend the scratch in, Autoglym paint renovator, Car Plan TCut/TCut metallic
Very deep but still not through paint - a more abrasive cutter, Car Plan cutting compound (almost solid stuff in a flat tin) or Mer Scratch remover (thick, in a tube)
If you are through to the primer or deeper (without rust) but the scratches are isolated and thin try Car Plan paint pencils, these are excellent and come in a pen like a felt tip but its REAL PAINT not like some crayon and pen maskers. Theses are not colour specific but the range has recently expanded to cover lights, brights, mids, darks, clear topcoats and metalics for most colours. Provided the scratch is not too wide you will never see it. (build it up above scratch then allow to fully set before using a cutting compound to bring it flat ) or very fine wet and dry - 1500 grit. Then a good polish to bring it back to a good finish.
Bigger scratches and rusted areas need to be fully prepared and the correct colour matched paint applied.
If you can find it (some Motor-Worlds still stock it - internal code 2607 000 0006 about 24.99, check contents before leaving store) then the Mer Scratch and Stonechip Repair kit is an excellent thing - if for nothing else than for the video you get with it!
Another thing - use a rough, slightly abrasive textured cloth like stockinette it will make it a bit easier on your elbow!
Good Luck,
Phil