You're opening the car door. You glance down at the fender and you see it -- A BIG SCRATCH! You mutter under your breath "THIS is why I can't have anything nice" as you look at your new car damaged before you. But fear not! It can be fixed.
Often times, your dealer will give you a bottle of touch-up paint with your new car, but this is NOT the time to use it. Touch-up paint is a bad idea 90% of the time.
10% of the time, touch-up paint is the right way to fix a scratch. If the scratch is large, if bare metal is showing, and if the scratch is a circular area the size of a fingernail, touch-up paint is the way to go. But I don't think anyone who isn't a pro should use touch-up. It's hard to apply, and most of the time it's too much.
The best way to fix a light scratch is called "wet sanding". Essentially, a paint pro will do an EXTREMELY light sanding on the scratch which will reorganize the paint/clear-coat molecules and make the scratch "disappear" -- provided of course the scratch isn't too big or too deep.
You can find out if your new scratch is big or deep with a couple of simple tests. When you gently pass your fingernail over the scratch, can you feel a bump? Does the scratch stretch over the entire length of the vehicle or of an individual body panel? If the answer to either of those questions is "yes", then you need to see a body shop and you may need to call the insurance company. Big or deep scratches can be expensive to fix, but not always. Make sure to check around.
If the scratch is light, your local body shop should be able to fix it up for a few euro. Make sure the shop you're going to us has a written guarantee and that you understand it. Usually the person repairing your car will tell you what it's going to look like when they're done.
One last thing - these are only methods to reduce or diminish a scratch, not to truly repair it. A true repair would involve a new paint job for the scratched panel and paint blending for the nearby panels, and I think that's a terrible idea to fix a light or even heavy scratch. It's better to fix these things inexpensively - they're just going to happen again. Just remember, the scratch isn't going to disappear. It will look a LOT better though.