How to improve my aim? (Xbox player)

To people with coordination trouble, how do you improve your aim? I like to play Overwatch but my friend keeps complaining (and rage baiting me) about my aim because it's bad.

I've been told to increase my sensitivity and that it will help me but I've been so so about it especially in heated moments or up close where I complete lose all ability to hit the enemy.

I've seen that it would be recommended for me to play with low sensitivity but I know everyone play at a fast pace and I'm afraid I'll slack off :/