Don't quickscope, it's hard to pick up and not something you want to do early on in the game (imo though).
Stick with Assault Rifles and SMG's, the Snipers are useful but MW3's maps are a bit too small to snipe effectively.
For classes I recommend using
Sleight of Hand -> Scavenger when you get it
Quickdraw -> Assassin when you get it if you'd like
Anything -> Steady Aim -> Dead Silence if you play SnD
Guns
UMP / M4A1 -> SCAR/PP901 or whatever it is -> Type 95 -> ACR -> MP7
^^ Those are generally the order in which I use guns when I'm leveling up, but use whatever you like, there really isn't any "bad guns" so to say.