The first thing I would like to talk about is aim. To become better you need to train your "aim" on various maps such as aim_botz. Try to aim at the head and not spray into the body, try with a maximum of 3 shots.

The second thing I would like to mention is movement. Movement is as important as "aim". Practice on surf or bhop maps. Of course, you can't forget about counter strafing (you can practice it, e.g. on the aim_botz map, try to have a crosshair on enemy head and move a-d).

The third issue I would like to raise is mental. If you feel like you're having a bad day, give up playing for a while, go outside and get some fresh air, it'll definitely do you good :)

The fourth way to get better is a team play. Without communication is really hard to win.
If you want to win more games, play with your 4 friends, this will reduce chances of getting a troll in your team. Teach yourself the positions on the map and communicate with your team.

The fifth way to get better is using a utility. If you want to get better, learn to cast your utility. Check out various types of tactical academies on different maps. It may take some time, but it's definitely worth it :)
(In due course, I will add a new guide where I will show useful grenades)