I'd suggest you main an AR and then have a shotgun or fusion rifle for close combat in smaller maps. If you're a good aim, a Sniper can be boss in larger maps as well.
Most of all, the trick is shooting for the head. It sounds obvious but you're really just wasting your time with body shots as you'll hardly ever kill someone with them. Unload an AR clip to the head and the enemy's dead before they react proper.
I'd suggest you start off in Control. Since the flags are in fixed locations and the indicator shows you when one is being captured, you get a good idea of where enemies are/are going. Modes like Rumble or Skirmish are a lot more hectic because you don't necessarily know what direction to expect enemies from.
After a while, you'll learn the maps, what routes enemies usually come from, you'll get grenade placing down. It comes slow, but it happens. I'm a fairly mediocre player at multiplayer shooters. This is the first one I've put some time into, and a week later I can say I've gone from being consistently the worst in my team to maybe second? Stick with it and git good.
