I'm not fully up to speed on Bad Company 2 multiplayer, as I've avoided the demo since I'm waiting for full release on March 2nd, and there are new modes I don't have any experience in.
But I'd definitely watch the "How not to be a Noob" video from PixelEnemy. Its really well done and covers a lot of the bases:
http://www.pixelenemy.com/2010/02/how-not-to-be-a-noob-at-battlefield-bad-company-2/
The only thing I'll add is that kills alone don't win matches. When you're attacking, make sure that whatever it is that you're doing, that it ultimately leads to your guys destroying the M-COMs, blowing up the gold crates, whatever. Hanging around as a sniper and spotting targets is very useful, but if that M-COM isn't getting taken down, switch gears and try to push the objective forward.
On Defense you need to be mindful of the M-COMs, but just straight up killing the enemy helps a great deal, since the attackers only have a limited number of respawn tickets. So let your bloodlust go wild on Defense.
One other defensive tip: remember that teammates can spawn either at their base or directly on one of their squadmates. So if you see a solitary attacker sneaking into your base, expect there to be 3 or 4 attackers by the time you get the drop on em.
Besides that, just have fun. Every time you get your ass handed to you, just add whatever technique that killed you to your own little toolbox of tactics. Then use those same tactics on the enemy. Every time you lose you'll only get better and better. Once you've got a real handle on the maps, roles, weapons, gadgets etc. the game will really open up for you.
