There are well over a million people that already have played Halo 3, and there were billions of games logged into Halo 2. But if you didn't play Halo 2 much, or just got Halo 3, I have a few tips that should probably have you being a better player. Some tips may seem obvious, but others may not. Some ideas may be ideas a "pro" would tell you, and some would be what a "n00b" would tell you.
1.) First things first, try beating the campaign on Heroic and/or Legendary. If you can't do it alone, ( Not many can. ) try doing it with only one other person. It will teach you how to learn strategies, and just overall get better at the game.
2.) This is an obvious one, but learn what gun combinations work best, and at what range. It should be pretty easy to figure out, and it shouldn't take you more than a few games.
3.) MELEE! - Meleeing is surprisingly overpowered in Halo and it should be used to your advantage. Learn to use it when you're up close to someone rather than shooting... unless you have a shotgun.
4.) Try to somehow stay in at least groups of two with someone, so in case one of your shields go down, the other one with full health can start firing on the other team while his/your shield replenishes.
5.) Crouch behind corners after running away from someone. You won't be picked up on their radar, and they will almost always run past you, giving you a free kill most of the time.
6.) Play Social matches when you first get online... Ranked matches are just for more experienced players, and it's harder to do well in.
7.) While using a gun other than the sniper, try to aim for the body rather than the head. The head may cause more damage, but it's a much smaller target, and it's just easier to shoot the torso. With the sniper, it definitely is better to go for the head, as it takes one shot to kill someone, rather than two it takes to shoot someone twice anywhere else on their body.
8.) Throw grenades! You might not feel natural doing it or something, but give a good strong throw, even at random times, and you may score yourself a kill. If not, you'll likely deplete an opponents shield, making it super easy for him to die.
9.) Get in a vehicle... this is one noobish tip, but it will score you a lot of kills, most of the time, if you're a good driver. Getting into a Banshee on Valhalla off the start won't do anything for you but kill you if you don't exit the cockpit soon enough, but if you're in a Ghost or on the turret of a Warthog, be ready to be getting quite a few kills... as long as you can avoid grenades and people hijacking you.
10.) The obvious, practice. Practice makes better, and if you practice a lot at the game, you'll naturally learn how to do well... unless you're flat out bad at videogames, then you're hopeless.
I doubt this will help much, but it's something.