-aim for the chest with BR, then shoot 4 times. Instant kill.
-Avoid the sniper till you get good, let your teammates who are better use it.
-Find a shotgun, and camp with it for some easy kills- shoot it, and then immediately melee/punch for a guaranteed kill from close range.
-keep moving.
-in free for all games, stay back, and take potshots on people with your BR, or throw grenades into the fray for some easy kill steals.
-vehicles usually get killed very fast in power weapon maps (Spartan Laser, Rockets, Brute shot), careful when using them.
-Dual weild a Plasma Pistol and a Spiker whenever possible, because this combination kills extremely fast.
Also, this is the biggest one:
-Campaign first to get used to gameplay, then move onto social games with teammates, then social free-for-all games, then to ranked. This will ease you into the gameplay.