I got lotsa tips for ya =)
1. Play for fun, not for stats. High KDR doesn't mean much in BC2, and since it's an objective based game, KDR usually implies that the person cares more about kills than about winning. (Except for a select few people that are just freaking good in every way)
2. Squad up. If you do, do you work together with your squad? and if so, do you use them as a mobile spawn point and do your own thing? or do you try to stick with them? One thing I always do when I'm not doing well, is find someone on your team that is doing really well, and join their squad. Follow them around, switch to a kit they are not (if they're assault, switch to medic, and vice versa), and help them out. It is really fun, because you feel like you're helping, and usually kills and points will follow =).
3. Duck, hide, and take cover! One of the beautiful things about Battlefield is that if you run out and try to mow people down, you will get mowed down and people will laugh. Find defensible positions with cover (crouch a LOT) like baricades and cement blocks. Building walls are usually fine, but they can get gustav'ed so watch out. Stay low, peek out, and sprint from cover to cover. If you are in a building, don't go directly in front of the window. Instead, stand back about a foot or two so people don't see you're gun sticking out, and don't stand in front of the window too long. Get up, look for enemies, and if you don't see any for a few seconds, crouch and move, then look again in a few seconds.
4. Watch your back. Constantly be looking behind yourself. When you go into a building, look through the rooms, and once you know it's clear, turn around and make sure no one followed you.
5. Use Ambient sounds to your advantage. This is a huge help due to DICE's beautiful audio. Listen for enemy voices. Characters shout things when they throw grenades, throw down ammo/health boxes, when they're reloading, and when they use the Q button to communicate. They also respond to things like when a grenade lands near them, they'll shout "GRENADE!" or "GRENATA!". All of these things help, because with the audio engine, anything you hear is direction-sensative. If you hear it above you, then they are above you; if you hear it in front of you and to the right, that's where they are. One way I use this is by throwing grenades in buildings that I'm considering entering. If there is an enemy, you'll hear them scream, in which case I take extra caution when entering the building and usually get the upper hand =)
6. Loadout. Mess around with kits to find out what you like. (I'm usually assault/medic, so all my suggestions are based on their guns) Do you like accuracy and burst firing? Try the M16/PKM. Do you prefer fast fire rate but less accuracy? Try the F2000/MG3. Love the big hitters? Try the AUG/M60. I have a couple loadouts that I really like. I'm an accuracy and burst fire guy. For assault, I love the AEK + Dot sight + Magnum ammo. From a distance, you can burst 3-4 shots, compensate for recoil, and get another 3-4 shots. If you are good at aiming, and due to the AEK's fire rate, they'll probably be dead after 2 bursts before they can even run. For long range maps I like the G3 + dot + accuracy upgrade. I fire off single shots from a distance and can get kills in 3-4 shots. For the more close quarters maps, I like the AUG + Dot + accuracy (or magnum depending on how well I'm aiming that day) due to its quick firing and high damage. For the medic, I use SAW + accuracy + dot sight and also use quick bursts. But it's all about your play style, so try different things, and eventually you'll find something that clicks.
These are just a few things, but if you work together with your teammates, you'll do pretty well =)
Good luck, and hopefully I'll see ya out on the Battlefield =)
