The main things I tell people are 2 things: Keep moving, and don't hold back. Literally every time I die and then respawn I'll just launch a grenade towards the enemy. I rarely get a kill with them, but when some guy is on top of you and you try to use a grenade I get kills even fewer times. And if you die and don't use your best abilities because thats "wasting" or whatever, then you really did waste them because when you respawn you get them all back. Also staying in motion. Diving, crouching and just running while hip firing is pretty good tactics for staying alive. (unless you snipe lol). It took me several years of playing fps to get good at them, but it'll come.
I have a 2.8kd in ww2