When I first started BF3 I was terrible. Absolutely horrible. Now I'm just mediocre but I am a special case. Most people with the hours I have in this game are very good so I wouldn't worry too much.
Whatever class you are playing at the time, look at your gear and see which items can help your team. Think for a second before you spawn in how you can use that item to best help your team. Take the ammo box, for example. Shortly after you spawn in, find an area with cover and drop the box. It will show up on your teammates' mini-maps and when they run low on ammo, they know where to go. As you run around, if you see teammates at a certain spot and the ammo icon over their heads is blinking, throw a box down (still making sure that it is behind cover). If you are trying to find an enemy and a teammate is jumping around in front of you brandishing his weapon in your face, drop a box. One tip, though; I used to chase after a teammate with a blinking ammo icon ready to throw down some ammo. He would never stop and I was too focused on him to watch for baddies and I would end up dead. Don't chase after him. He hasn't run low enough on ammo that he cares yet, and your help could probably be used somewhere else.
Every class has some useful items. Think about the best way to use them to help your team and do your best to do that.
Oh yeah...one more thing. Stay relaxed. Have fun. When I get frustrated, I play so much worse. Once I take a deep breath and change my attitude, I start to do a little better. Instead of getting mad, think about why that player just killed you. Was it pure aiming talent...not much you can do. Was it because he seemed to know where you were going to go...figure out how he knew and try to incorporate that into your game. Did he get you because you did something like run at full speed around a corner into a whole squad...well, what could you do next time at that same corner to give you a better chance. Are people always shooting you in the back...remember to look back and watch your mini-map while moving deliberately (not necessarily at full speed).