How to better my aim at COD?

Steel Series Rival 700 The Only Mouse That Vibrates. Best for MMR

What gaming mouse is better for Action Games? I tried different variants. Razer, Logitech, A4Tech Bloody, X7... They all good, but X7 is better for me.

Pratice shooting and being ready. It can take time

Well my bestsuggestion is pratice makes perfect when it comes to Call of Duty. As most of the games remain the same so if you dedicate your time to it you can be succesful.

I used to play custom games with friends or playing zombies to help improve my aim. also the campaign should help.

I think it's very nice. However, it costs just under 50 and is therefore a bit more expensive. There are also in-app purchases.
Graphics, etc., is of course nice

it takes a while practice makes perfect, prac the recoils and fidn your favourite guns

If it's the PC variant you'll also want a 144hz monitor and high end gear or very low settings to run at the highest FPS possible. This is for the smoothest aim and frames, and also to help with trickier shots. Bump up the digital vibrance if you have to, turn off anti aliasing so the outlines of people are more jagged so they stand out more, Some peole even play on 4:3 just so people seem larger. But that should help a ton.

I find Osu! helps me with a lot of FPS games, really improves my mouse precision

First is equipment, but the most you need is reflex. If you can good aim at body/head try the tactical training on cod. And try to shoot fast on the fields.