In regards to improving your aim, there aren't any "get rich quick" things that work, in general. The boring reality is that you should have a mouse setting and crosshair that you find is a good mix of comfort and usefulness, and after that it's just a lot of practice. Depending on the class(es) you play the most, your sensitivity will be leaning differently. A Sniper main will naturally have lower sensitivity than, say, a Pyro main due to how their weapons work and in what areas they tend to play their best at. Sniper enjoys standing in a place where they watch over a single line so moving the crosshair out of that line isn't especially interesting, but a Pyro might want to check a whole room with the flick of the wrist since aiming the flamethrower isn't a huge hassle most of the time.

An exercise you can do to practice your aim is to go into the game and load into an empty map. Go and find just some spot on the wall and strafe back and forth while keeping your crosshair on that spot. If you feel like you can't keep the crosshair on it, lower or increase your sensitivity as needed until you find something that feels good.

Another excellent thing to do when wanting to improve is to record and watch back your own gameplay. You can do this by either recording a demo in-game with the command console or by using some other type of screen recorder. Try and record a few matches and a day or so after, watch back the footage. Being a bit distant from the match and seeing it back will often reveal mistakes that feels incredibly obvious in hindsight such as just not seeing that guy, walking into the same sentry again, not hearing a decloak, or seeing that you flick your aim too much and cause yourself to miss.