Okay, first of all get Xbox Live. Then go into ranked matches, there you play people of your skill level and you get better.
Here are some helpful tips on getting better at Halo 3.
When using a rocket launcher, aim at you targets feet. This will inflict splash damage. Also aim where you target is going to be, rather then where they are.
If your target is around a corner try and throw a grenade at a wall so it rebounds near your target. Also use sticky grenade and throw them directly at your target because if a sticky grenade sticks onto a target its 1 Hit K.O. sticky grenades are Plasma and Spike grenades.
When up close to a target melee them while they're shield is down. This will guarantee a kill. Melee is by default on your controller, B.
When using a Battle Rifle aim for your targets head. This will inflict maximum damage to your opponent. When using and assault rifle, plasma rifle, SMG, Spikers or Needlers aim for your targets body rather than they're head because it is not guaranteed to hit.
Also when using a plasma pistol try to dual wield (use two weapons at a time) with a normal pistol as well. This is because when you charge up a plasma pistol to the max and fire it at a target it takes down the shield, then shoot with your normal pistol. This is guaranteed to kill your target.
Try to get into the habit of reloading regally so that when you do come in contact with an enemy you dont have to reload because you become completely vulnerable.
When using a sniper rifle try to Snope you target (no scope head shot). Although this is a technique used by only the most skillful snipers getting into the habit will be very useful when you are under attack by several enemies.
You may not know this but when you crouch or stand right up against a wall you are not visible on enemy radars. This is useful when you are being a sniper or if you are going to assassinate a person (go up behind an enemy and melee them on the back, its 1 Hit K.O).
When you are being shot at and you dont know where they are or they are not visible when you look around they are either snipers, or they're invisible or they are crouching/against a wall. When this occurs you should never stop moving, just keep looking for them and jump jump jump and keep moving left and right up and down. This will make it difficult for you enemy to aim at you and get a good shot at you head.
If they're is anything other that you dont think that i have covered or i have not explained clearly, please e-mail me.
Also watch video's on Youtube because they often have video's with professionals playing against each other. You can learn alot from watching them.
You can also Google Tips on playing Halo 3.
