Hey i felt the same way until i started understanding the game more. Heres some tips..
1) Dont just run in like a mad man shooting, try to wait for the enemy to run by then sprint behind tem and blow a shell or round in their back.
2) Aim for the head. 1 shot shoots off the helmet, the second shot kills or deals like massive damage. (can kill 1 shot)
3)Sometimes people are cheating because the game is lagging an become invincible gods. When this happeneds vote to boot the player and if the boot fails leave and join a diffrent server.
4) Find the weapon that suits you best.