Kill yellow
Green is your squad
Blue is your team but not your squad.
You heal by dropping med packs. You will have to unlock this. It's quickly though. At first, just kill people, take objectives and join a squad. You'll level up in no time.
Then you'll unlock your paddles and can revive. This is how medics rack up points quickly. If you do nothing but squad heal/revive, you'll rack up the points.