Tips for beginners:
Always follow the leader or the one who has highest experience in this game.
Do not wander away from the squad, but if you know what you're doing, go ahead
Try pushing the arrow keys for some radio commands especially the Help Me button if you got in some trouble.
Try to learn while you play along.
Use you're medkit if your hp is below 20 - 50 %
Only use pain killer in emergency, although if there's nothing going on, go ahead use it
Try to use interact, it can give you some useful info.
Use crouch if you're engaging in battle, it gives you 50% damage reduction!!! with also +10% damage!!! OP much? but with slower movement speed.....
Only use grenades if there some bunch of enemies, also use it against boss, aim it carefully since it bounces when you throw it depending on how far away you threw it.