Also, one thing that helps me out is getting on YouTube and watching videos about the character classes. it helps me understand their abilities and gadgets.
For example now I'm watching a video on the Recon's SOFLAM targeter. Just planting that in a busy area might help the team by designating targets and getting assist points.
One more thing is everyone should learn about spotting. That puts a marker on a target so your teammates can see them. If someone else kills them you still get a spotting bonus.