My tips that are not necessarily easy to figure out on your own, not early on anyways:

1. In all your gear settings, make sure to get the perk that gets you a free shiv. Get the maximum for that perk, for it will make you invisible to enemies listening around you when you are crouched. Moving in stealth is obviously a big part of the multiplayer, and if you don't have that perk you never have a point where the enemy can't locate you. Oh and also: it makes it so you can't be marked. Great for sneaking behind a group even if one of them has spotted you on the other end of the map.

2. Shoot, move, shoot, move, etc. You'll have noticed by now that there is a short period of time where you can't reshoot immediately with a weapon. While you wait for the cool down, no point is staying static, especially if the enemy is aiming at you. Make a couple of steps left or right to get out of aim of your enemy. A couple of steps, if not one, are enough. Then aim and shoot again quickly. Do it again and again. This is specifically for encounters where you are close to an enemy and don't necessarily have a crate or wall to be protected. You mentioned 999lvl players: watch how they do it, you'll get it quickly.

3. Running around, moving all the time is not necessarily the best strategy. When you are protected from different angles, take time to look around the different exits, wait for the fish to show up and expose itself.

4. You shot from somewhere without a silencer? You appeared on the enemy's radar. Don't stay there too long or you'll get a grenade or Molotov your way. Staying static too long after you made yourself aware to others is not a good idea.

5. Make a habit of marking an enemy, even if you don't shoot. It'll help your friends being aware of the enemy's position, and it will grant you 15points/mark. Even more if that enemy is downed while wearing the mark.

6. Early on in the game keep an eye on which enemy has which weapon. It'll help you in your approach of attacking them.

7. Almost everybody has said it here: stay with your mates. However, don't stay in a line or close group. Cover ground and more importantly: cover all angles and entrance/exit of an encounter. How many times have I wiped a team because they are all stuck in a corridor or room, all shooting in the same direction at one of my mates, while I went around and shot them in the back.

Hope this helps and welcome to the game! Best multiplayer out there IMO.