While playing th game you have to press this button ¬ then it a chat box will appear. Then you can type one of these:

Add cpu characters:(you need to be the server to do this cheat)
/addbot1 = Terrorist
/addbot2 = Counter terrorist

increase or deacrease gravity:(you need to be the server to do this cheat)
/sv_antigravity (maximum number 0-9999)

kick player from the server: (you need to be the server to do this cheat)
/kick (type player's name)

kill player: (you need to be the server to do this cheat)
/kill (type player's name)

speed increase:
/cl_forwarspeed (maximum numbers 0-999)

homming bullet
/sv_clienttrace (9999) (you need to put /sv_cheats 1)

and walk in air:
/sv_airwalk (9999) (server can do this cheat)

gl_maxsize 99999
lightgamma 0
brightness 99999
r_wadtexture 9999
sv_sky_color b 999999

sv_clienttrace (9999)

only the server can actevait this cheat

and only after actevaing the sv_cheats 1

and the restart

after the sv_clienttrace is actevaited you can shout next to the other player and kill him

Teleport

will work in other level but also work in De_Dust in the begining of the game

when selecting you terro. or Ct skin don't select anything yet just press I and 0(zero) and coose auto-select player

and presto