1) When shooting from distance, don't hold down the shooting button, instead, tap the shooting button while crouching and aiming at the head.
2) Use the walk button! It great when playing games when there's not many people on. When you walk (Hold down shift normally) it makes it impossible for others to hear you!
3) Use crouch! When you crouch, your aim becomes a lot steady and makes it easier for you to head shot the opposition! It can also confuse your opponent if the think you are standing up when your are actually crouching!
4) When knifing, use your right-click button on your mouse to attack! If you use the left-click button, it is called slapping and is very weak and can take up to 6-7 hits to kill someone, if you right-click which is called actually using the knife correctly it would take 2 hits to kill someone!
But I would tell you the guns you should not buy!
1) The first pistol on the pistol menu (The gun terrorists have for free)
2) The first rifle on the rifle menu (Only available to terrorists)
3) NIGHT VISION!!! NEVER EVER BUY NIGHT VISION!!!
4) Machine gun which costs a fortune and it's so hard to aim with but when you throw a smoke grenade, you should shoot randomly and you should get few kills.
With this information you would improve heaps and might become a pro!
5) If you can't shoot without the cross hair or reticule (the + in the middle of the screen) don't use the sniper rifles