1. Learn the weapons, figure out which ones suit you best. Some are better with short range weapons, some with long range. Try out a bunch but once you have found the right ones, stick with them. Switching weapons too often can cause confusion, learning the exact time to shoot or reload takes time.
2. Don't use rocket launchers, grenade launchers or bulky explosive weapons. You may be able to rack up some quick kills, but everyone will hate you for it and you will be considered a noob.
3. Get to know the maps. Find out where the grenades, special weapons and key battle areas are. Wandering around aimlessly is a sure fire way to get picked off by a Noob Pwner. Not knowing where key items are will put you at a disadvantage against competition. The early bird catches the noob.
4. NEVER run out into an open battlefield. 99.9 percent of the time there is someone waiting in a corner or aiming at you with a long range weapon, waiting for the first chance to take you out.
5. Choose your battles wisely. You may see what appears to be an easy target but chances are he's either not alone or has protected himself with some sort of plantable explosive or trap. Use flash bangs and or explosives to take out unsuspecting enemies. Be aware of your surroundings when entering new rooms.
6. Customize your weapons according to how you like to play. If you are a run and gunner a red dot sight might suit you best. If you are a sniper or prefer long range battles add scopes and attachments that add distance to your weapons.
7. You see those guys carrying guns next to you that aren't shooting at you? Those are your teammates, stick with them, you're stronger in numbers than you are alone. The lonely wanderer is more at risk of being picked off by multiple enemies.
8. Kill/Death ratio is important. You will be ridiculed for anything under a 1.0. Monitor your deaths and don't be afraid to put the controller down and take a break for a while when you are on a bad streak.
9. Help teammates by laying down cover fire or by picking off enemies getting to close to them but NEVER run out into the open to pick up a teammate. If your teammate is within reach and you can help, do so. Otherwise there is no point in both of you getting killed.
10. Don't attack heavily armored enemies or enemies carrying dangerous weapons up front. Try to find an alternate path, sneak around and wait for the right moment to attack.