I have just got a new guitar and it had stickers on it,
after i took them off, left behind was a sticky mark.

its pretty hard to get off and I was wondering what i should use to clean it
This stuff called Goo Gone worked for me... there's a bunch of products like it, check out your hardware store. I'd make sure that it won't mess up your finish first, though.
Quote by blink rock hard
Its only on the scratch plate,
I dont know wether that makes any difference

If it's a common body style you can probably just get a new one if it messes up the finish... hell, you might want to just do that in the first place.
I don't know about guitars, but I've used peanut butter to loosen up the sticky stuff from price labels on other stuff. Should work for anything else.