Killjoy new trick

So a week ago I bought a new monitor which is 240hz but the second I started playing my aim just got worse. Does someone have any ideas why that is

I’m ver new to the game and I wanna rlly know who to get to be one of my first few characters i like to be aggressive but my aim isn’t that good and I need a lot of heals and recon so who do I choose I’m trying to choose between Reyna and f...