One of the most important features of the game is that you must play your class properly. You cannot expect to be a medic and run around racking up kills by yourself, you will have to help heal your teammates and learn to work together in order to accomplish the goals of the map. Battlefield 3 has set the standard in first person shooter games, and with it comes the requirement to actually play your class properly, or you and your teammates will die quickly. So for example. if you are an engineer, be sure you are repairing tanks and shooting enemy planes and helicopters out of the sky.
Another important aspect is the controls of the game. Some people prefer an inverted airplane style play so that when you pull down on the control stick, the view looks up, or if you push up on the control stick, the player view looks down. Be sure you try out both methods to see what best suits your style of game play. You may be surprised at your findings.
An excellent way for you to get some extra kills is to change up your game play style. For example, I found that when you come face to face with another player, instead of moving back and forth from side to side trying to avoid their gun fire, immediately drop into the prone position and aim for the head. This type of tactic works very well because most people are not used to facing an enemy who does this and gives you that extra bit of time to get your head shot in while they are confused. Ultimately this second of time will be a huge advantage for you. Try it and see for yourself.
What I would consider the most important Battlefield 3 strategy would be to get yourself a good strategy and tips guide. This will greatly improve your game as well as your experience. There are many types of guides out there, I personally prefer a guide that offers HD videos so that you can see exactly what tip or techniques is being described. There is no better way to learn than to watch someone else do exactly what they are describing on video for you to watch and imitate.