Tips for someone who is not the best in this genre

I've played a fair bit of shooters in my day, from Serious Sam and Unreal Tournament back in the day to Overwatch more recently. However, I've never exactly been the best at these, so as someone who is extremely interested in this game and wants to be at least decent at it, is there anything I should know beforehand? Any sort of practice I could do to try and improve myself for this game?