I really suck at this game. What can I do to get better?

I haven't ever played an FPS before, and so even the basics of shooting an enemy I fail to grasp. Playing Overwatch is the most fun I've had playing a video game in years but I have to keep the fact that I suck in the back of my head. My weapon accuracy is always below 25, if someone sneaks up behind me I die before I realize where I'm being shot from, and damnit if I walk into another roadhog hook I might AFK. I'm level 85 and I've only gotten 2 POTGs ever!

I get flamed a lot in quick play so I avoided competitive because I didn't want to hold my teammates back. Advice? Thanks :)