It may be bugged out. But if they have a little blood icon, use the bandage to stop the bleeding by holding left click and letting the animation play through. Once a medic symbol pops up, use the medic bag by holding left click to get them back to full health
It'll take a few months to get any good at it. Jump in a squad with a TBG member and tell them your new, you'll learn quick. Just do as SL says or you probably won't be that squad long. When you get sworn at just remember it's nothing personal.
Yea biggest things are 1)letting your sl know you are new, and 2)dont auto lock marksman. Acog rifleman is probably the best kit when starting out, you get a magnified optic, but are still playing objectives and overall just helping out your squad.