How to better my aim at COD?

If it's the new Modern Warfare, it depends. Console version, I'd say change the thumbstick setting to closest, or linear, basically any other option than default makes the deadzone and aiming in general smoother. Next is of course the sensitivity, now this applies to pc too, as you have to find something you're accurate with and consistent, then stick with it, build muscle memory.

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.