If you are dying too fast you might be ADS'ing too much or maybe standing still too much.
You should always strafe during fights and if you are in close range you should avoid ADS'ing as much as possible and toggle it when needed. When you ADS you move with reduced speed so it is easier to one clip you.
For example you can watch the following video for strafing:
How To Strafe Like A Pro On Apex Legends (Movement Guide) - YouTube
It is just an example video so you can probably find better ones.
You should also have to use cover as possible even in close range and try to bait enemies to waste their clip by strafing behind a cover. For example you can stafe out from a cover and most players will start to shoot and since SMG's have fire rate if you go behind the cover fast they will waste most of their clip so you can shoot them back without big damage.
