FPS Aim is a very well documented field now with loads of knowledge and an equal amount of BS. The first question to ask yourself before diving in here is: Have I looked at the other sections? Aim is important but if you are just starting out, there's a high probability that you'll find faster gains from other parts of your game. As a beginner, the best thing you can do for now is just make sure you have a comfortable mouse with a good sensor, a large clean surface to mouse on, and a sensitivity somewhere between roughly 25cm and 35cm/360. Once you can physically aim comfortably, you can dip your toes into aiming theory, notice the differences between aiming with movement keys and with the mouse, differences between aiming at targets above and below you, and the difference your grip makes in your mouse control. Just remember two things:
1) The goal of aim training is to make aim intuitive so you don't have to spend mental energy thinking about it in game.
2) Aim never tells the whole story. The cause of poor aim is often not related to mouse control. Your gamesense, positioning, weapon selection, mental, diet, sleep, posture can all make your shots easier or harder. Aim training can help you hit harder shots and hit easy shots more consistently but it can't help you understand why you always end up taking hard shots instead of easy shots or even why you always miss that one easy shot.