If the scratch is just a light one try using T-CUT, it will make it less noticable, if the scratch is through to the primer or bare metal go to halfords or any car shop that stocks car paint and buy the right one for your car, then spray 3-4 light coats 15 mins apart, leave the paint to fully dry (about a week) then use 1200 grade wet and dry to smooth off the finish, then use t-cut to polish it off.
Check what your paint code for your car before you buy the paint, it is normally printed on the inside of the door and will on most cars be 3 letters like KRB