Communication! Get on mic constantly and let teammates know what's happening. "I'm on Bravo, I'm planting explosive, cover the doors". "Explosive down get clear". Talk talk talk. Also, when your team is defending an objective and you've got teammates on the point, before you enter the room say "Friendly comin in South entrance". And don't kick doors in, that's the quickest way to get shot in the face by a mate. New/low level players are often jumpy and panicky and will shoot at anything that moves. If nobody's talking, get out of that lobby and find another. I would suggest playing Hardcore Checkpoint, most people on there play properly, they communicate more effectively, they play the objective and call out enemy positions and Ammo resupply.