If you're coming from BC2, a couple of things you should be aware of..
1) Prone and strafe runs are back
2) Rush is nowhere near as good in this as it was in BC2, conquest is where it's at (chainlink is a bit like a fast version of conquest).
3) The destruction is not as great, you can't completely flatten every building, and it can be frustrating to start with learning what you can and can't demolish. For instance you wouldn't expect to be able to hide a squad in a shipping container in real life and survive direct tank hits or a cruise missile.
The starting weapons for each class are actually pretty good, but ignore their stats in battlelog, go to symthic.com to see how they really compare, and practice in the test range to get your optimal loadout. Note many attachments that improve accuracy or recoil now have detrimental effects on spread or spread recovery, so if you're not careful you can make a weapon worse.
Try just being a good squad member to start:
1) Get to a safe position near an objective and try and get people to spawn on you to take it.
2) Play a good medic with your medi pack and defibs
3) Play a good engineer with by finding a squad vehicle to gun for and repair, and harass enemy vehicles whenever possible.
4) Be the ammo and suppression guy as support
5) Don't go recon and camp in a far off hill
Remember to spot, and use the game setup menus to increase your mini map size and keep an eye on it for the enemy.
Don't start firing as soon as you spawn on a squad mate unless you have to or are using a suppressor. You'll probably get yourself and your squad mate killed when the enemy see you on the map.
Even if your camping, always keep moving, or else the pew pew brigade will get you.