Before you attempt to polish the scratch, try this test:
- run a fingernail lightly, backwards, across the scratch
If you can feel the scratch then you'll need to get it professionally repaired by someone like Chipsaway (if you're in the UK).
If you cannot feel it then you have a couple of options. If it is a very light scratch then try Autoglym Super Resin Polish. Use a polishing cloth and some firm pressure AFTER you have thoroughly washed and dried the area.
If it's a more noticeable scratch then you'll need a much more abrasive polish like Autoglym Paint Restorer or T-Cut. Use these with care as you can end up ruining the paintwork! Always ensure the area to be treated is clean and dry before you attempt a repair, and always use the 'nail test' to see if a scratch can polish out. If you can feel the scratch then it needs to be filled in professionally.