The game movment is sh.it, so that's make your aim so bad.
You need to play with low sen to get a time to aim at the head.
The game depens on headshots only as i see.
I'm a legit plat 3 peeking diamond 1 right now, and i think the best and also the fastest way to improve is to go on aim lab to train your aim, deathmatch is also a great choices. And about decisions making... you just have to play more