Most of what worked on BF2 works on BC2. Notice I said "most" and not "all" as there are differences.
Starting out at zero all you can do it go out there and kill and be killed for a few rounds. Once you get the first unlock or three you can begin to act like your actual class. Chaos's advice is very true regarding Medics and med packs, and the same goes for Assault and ammo bags.
Medics carry the big guns and can lay down a barrage of cover fire as well as chew up protecting walls. Remember that. Also remember that the reaction time on a big gun is slower than a smaller one so try to engage at a medium distance.
Assault work good for medium to short range. Ammo bags are good for points, especially within your won squad. Noob tubes come in handy once you gett he hang of aiming them.
Engineers, while geared toward vehicles, can be used elsewhere as well. Their RPGs make good anti-infantry weapons and can take down buildings with a few good shots. You can also kill with the repair tool and I find this more disgraceful to be killed by one that to be defibbed. An engineers gun(s) are pretty weak compared to others but wok well in close.
Recon (sniper) are long range specialists, but are fun up close once you unlock a shotty or two. Mortar strikes (once you unlock them)can really give the opposition a rough day and can take out a tank. Don't forget to toss out sensor mines if you have them to get spotter points and to help your teammates.
Speaking of spotting, spot any time you see an enemy so they show up for friendlies. That goes for a lone gunman, a tank, a heli, or whatever. Some get annoyed by it, but they will still use the opportunity to blow the enemy away.
Finally, hit YouTube for loads of advice and tactics. BC2 is not BF3, but it is loads of fun in the meantime.