I'm actually a pretty decent Sniper. Out of all the classes, he's my favorite. But I also play a MEAN Medic.
The key to being a good scout is to have good reflexes. And also to try to sneak up on people and blast them with the shotgun from a close distance. The closer you are, and the more in-center you are in relation to the target, the more damage your shotgun will inflict.
If you play on the PC, try looking for some newbie servers. I dunno how it is on the 360. :)