Combat lesson one: aim for the head. If you can't do that, aim for the nuts, it hurts as much as the head.
Combat lesson two: buy a gun! Ammo too you stupid! oh..now you blaming me for getting youself kill?! It was you who teamflashed yourself! And buy an armor you moron!!
Combat lesson three: chicken = bullet in the head.
Combat lesson four: try to use "cl_crosshairscale 10000" in the console. Trust me, it helps.