Wow, what a load of extremely useful advice for those who are already pro lol
For starting out, I'd suggest playing the classics, like Doom,HalfLife,Quake,CS,Battlefield.
Also Destiny 2 is pretty much a shooting gallery, and while it's an online game, it's mostly single player content and gameplay actually, so there is no pressure there.
I'd say playing ARMA3 for a start will put you off of fps games for life lol.
Here's a link to a really fun complete doom thingy
https://www.moddb.com/mods/recurring-nightmare/downloads/brutal-doom-hell-on-earth-starter-pack
Give this a go at the easier difficulty levels, than jump up the difficulty to Realism mode. It's very hard to stay alive, but if your aim is good you can kill most tthings with 1-3 shots so it is immensely rewarding.
It's a complete free game, but you might need to turn on the reticle and change some of the keyboard layout in the options.
Situational awareness is the most important thing to have. Always know your surroundings. There is only one real way to improve this. Play. Play as much as you can. Some basic skills include :
Moving in the direction you want to while aiming in another direction
Strafing around corners and basically everything
Hugging walls and already aiming around the corner before coming around it
Reloading at the right moment, in cover preferably or on the move
Tldr: play the classic shooters (not serious tactical shooters like Arma) as much as you can, you will pick up the basic skills.