Depends on which Halo they all come with their own lil changes to how they play accuracy,aiming,mobility,weapon strengths and so on. While I'm by no means a great or probably even good player I find that playing without worrying about your KD helps a lot I've hit the slicks game type this weekend and not worrying about wether my KD is going down has helped me relax while playing. After my last social game I went on to have a game of WZ and got 7 boss kills 24 Spartan kills and 6 deaths while not as good as many out there it was good for me.

Play Rumble Pit, get used to fighting more then one opponent. It'll make you a better player for sure

Use a mic and play with a Fireteam that has mics so you can use team cooperation.

You don't want to go running into firefights guns blazing. Stay back with a pistol or other somewhat ranged weapon and take on the enemy from afar. [u]Only[/u] attack close up you can either a.) get the drop on them from behind with a melee b.) can use cover to move in on the enemy and use a closer range weapon or c.) if you are with teammates and can have them distract the enemy while you move in for the kill. Don't go running straight through the middle of the map in BTB or big slayer maps and if you ever decide to play Breakout remember: you don't have shields so you can't go running to an enemy and expect to not get killed by a pistol, DMR or BR headshot because those things can two shot you (three shot for DMR and pistol). Oh and don't be afraid to use the noob combo on enemies. (Plasma pistol and a pistol or other precision weapon.) Edit 1: Only go for the power weapons if you know that you can get them without dying. I've made that mistake far too many times.

Hang back, steal everybody's kills. Always has and always will be the way to win in a Halo game lol

Grenades aren't to kill as much as to get the enemy player to move to where you want them

Lots of practice. Believe me, it helps

If you wanna play with me and my group feel free to add me. We are going play little warzone tonight my GT is the same as my name on here

Power weapon control, map control, [i]decent[/i] aim, and picking your fights.