Free tips:
Stretched resolution
Stock mouse dpi
Sensivity lower than 2.7 in game. (Pro setup, 6/11 windows, pointer accuracy off, acceleration off, 2.5 ingame and brute entry mouse)
Play with stretched resolution (easier to aim at head)
Play deathmatch for 200 kills before any MM.
Play with an headset.
Talk with your teammates (you will loose all your match if there's no communication)
Press shift, never crouch when firing at someone just straffing left to right fastly and shoot always few bullets per few bullets and never full spray, learn weapons recoil offline.
Never use cheats or you will loose your skill!