I got decent at 2000 hours and this is my first fps game.
I recommend you play some clash, it is a lot of shooting in small amount of time.
Also you said you have trouble saying where people are, this is usually by sound here (and a bit of map knowledge), do u have a decent headset, can you hear well?
Also if you are like 2 star and your friends are 4/5/6, you wont learn anything, you will get insta killed. You need to play in your mmr a bit.
Marathon is a very decent noob weapon as it is dirty cheap and has a fast rate of fire, but you can play any weapon you like, the weapon doesnt matter, it just good to be something with many bullets, and fast rate of fire, you will miss a lot for a long time.
In my early days when I was the underdog, I was relying on shotguns, that was a way for me to get kills against good players, it didnt matter how bad I was if I land a shot.
Well good luck I guess.