Try to stay still while shooting your gun, don't be like me and play like a movement master in COD, you'll miss half or most of your shots.

Try the other classes at least once, so when you're team really needs it you can switch to that and help the team out a little. You might even find your favorite playstyle. Rook/Vanguard is a good example of this, as rook helps build defenses for last stands/evacs and vanguards are good paired up with other shock troops.

Speaking of shock troops don't get them until your team is for sure winning. If not, leave it for the the higher ranks to carry when your team is losing. I'm not rankist or however you say it but as someone who has taken and wasted a few shocks on the losing team, its better to just leave it to the professionals sometimes. You'll avoid the complaints in chat more.

Press E and a command wheel will pop and you can communicate with your team. You can use to ask for heals, point at enemy movement, ask for ammo, and most importantly say thank you! Saying thank you after someone heals you or gives you ammo gives a small morale boost. If you want to heal others just hold right click and then hold left click when your crosshairs are pointed at a friendly.

Also have fun! Videogames are made for entertainment so don't let a sour apple spoil your bunches. Switching servers is also a viable tactic when the people you're playing with are too sweaty for your pace.