Battle royale is not the best kind of game to practice FPS as a beginner, because actual gun fight time Vs looting and running can be sparse. My advice is buy Titanfall 2, it's currently very cheap, it has the same gun mechanics and when you die you instantly Respawn, there is no looting only constant gun fights, there are even enemy minions in the match that are easy to kill so you won't feel completely useless. If you can't afford it, or simply don't want to, you can play free to play FPS games like Team Fortress 2 to practice.
Now in terms of mouse sensitivity, what sensitivity are you running the game at? What DPI setting is your mouse at? Have you disabled mouse acceleration in Windows mouse settings? Are you wrist aiming or arm aiming?
When I first started playing Apex I was really really bad, hardly getting any kills, usually dying to the first player I ran into. After watching some YouTube videos I realised my DPI was too high. I was running at 1200 but every day would drop it by 200 DPI, I'm now currently at 400 DPI, I've had to buy a bigger mouse pad and I am now using my whole arm to move the mouse instead of just my wrist. It has taken a bit of getting used to but I am now finding it much easier to consistently hit targets now.
When your sensitivity/DPI are high you have much lower room for error.