Huh, didn't see this yesterday.
What kind of guitar is it? Because it really makes a difference. Different companies use different kinds of finish, so it is best to know what was used, which can usually be determined by the brand and age of the guitar.
The basic method, if it is a very light scratch, is to first level the finish (usually using a very fine wet or dry sandpaper - 600 to 1000, used wet). Then, you polish it out with a series of polishing compounds, usually automotive type compounds. The thing is, it really helps to have some experience with this stuff, because you can sand through the finish and make it worse.
If it has turned white, then it is PROBABLY too deep for a simple fluff and buff. At that point, you are usually looking at needing some touchup to get the finish level, and this is where I REALLY need to know what kind of guitar we are talking about. If it has a nitro finish, it can be repaired, but you really want to let a pro do it, as nitro is very toxic stuff, not to mention explosive. And the chemicals used to soften the surrounding lacquer (thus avoiding witness lines) is even more toxic. There are guys who would suggest a French polish touchup, but it would never even look decent, much less right (at least, to my eye). Certainly, it would never look "mint" again.
Of course, if it is not a nitro finish, all bets are off. Your are simply fucked if you can't get there with wet sanding and polishing.
Which brings me to the really good news. Even if it is nitro, you may well be fucked. If the guitar is more than a year or so old, the touch up will always show. We can do a lot to minimize this, but as lacquer is exposed to UV rays, it yellows, and that color is one which simply can not be recreated. Believe me, some of the best in the finishing industry have tried, and it simply can not be done.
So, not much in the way of good news, I'm afraid, but there you have it.
Light
"Cowards can never be moral."
M.K. Gandhi
