This instructable will help you understand some of the better ways of playing and give your the help you need to win.
Step 1: Know Your Controls
Obviously skip this step if you know how to play. If you press start and go to the right there is a option to configure controls. Change the controls until it feels comfortable. Here are a few basic tricks to give you a bit of help. Use the melee control often as it can not only save ammo but it can save your life. While jumping will not save you from a persistent enemy it can bring you into a tactical waiting spots
Step 2: Know Your Map
Before going on to matchmaking go to custom games and just run through the maps. Each map has its choke points (places were people can only go in one direction), its sniping points and so on. Also keep a eye out for weapon placement and think of how you could use that to your advantage (ex: on construct there is a flame thrower at the bottom of gold elevator). If you look really carefully you can find ways to get into good hiding spots. Return to the map on forge and find out all of the respawn points.
Step 3: Know Your Allies
They made voice communications for a reason. Keep your headphones plug in and be a active speaker for team tactics. (Just don't spend the match singing instead of playing). When it es to talking keep this percentage in mind. 50% of your talking should be about strategy, 25% about gloating, 25% about random stuff. Also use keywords to aid in strategy. One example is using codewords for areas on the map(On last resort "guy in the sniper tower').
Step 4: Know Your Enemies
When the match is going underway look for patterns in what your enemies do. If they always go for the sniper rifle stay in close quarters. If they always go for the ghost grab the rocket launcher and go to town. if they like to run straight for the flag stay put and defend it (It is not camping when you are defending your team's objectives)
Step 5: Know Your Weapons
Every gun has its pros and cons. And while a magnum will never be equal to a shotgun some weapons have advantages over other weapons. While a rocket launcher is better against a vehicle a battle rifle is better when it comes to fighting against a lone person. Always remember the best ways to use weapons. Don't try to use a sniper while indoors or a flamethrower while outside. Be smart how you use weapons. Try to have one long range weapon and one medium to close range weapon. Also use your grenades wisely. Each one has its advantages over certain situations. A frag grenade is good in a close quarters, A plasma is good against vehicles, a spike is good against sticking a person and a firebomb is good for a group of people.
Step 6: Know Your End Result
After a match press the back button on your control to bring up pregame result. With this you can see who you killed, who killed you and other important things. First look up your name. In the middle you should see a black square with medals in it . You earn those by doing certain things(splatter someone, snipe someone, kill two people in 30 sec). The bottom left has your number of kills,deaths and your K/D ratio.The Bottom right has who you killed most and who killed you. The bottom middle has you tool of destruction(what weapon you killed the most people with). back away from your name and look at your stats for the match. They will help you compare yourself with the other people.
Step 7: Final Notes
these tips will not give you a guaranteed win every game. They will however get you started and well on your way to being good at halo 3. You should also consider making a bungie profile. It will let you understand more of your abilities.