Enable the console from the game's in-game options. Hit the tilde key [~] to drop the console.
type seta thereisacow "1337"
now type "spdevmap"
an error msg will appear, ignore it. Klick END and resume the game or start a new game.
Now in the game you open the console with the [^] - key again and you can type in the cheat codes here.
give all - Access all weapons
cg_LaserForceOn 1 - Add laser sights to all weapons
jump_height # - Adjust gravity default = 39
timescale # - Adjust time default = 1.00
r_fullbright - Easier to see things
notarget - Enemies ignore you
give ammo - Full ammo
god - God mode
demigod - God mode but screen still shakes
noclip - No clipping mode
cg_drawGun - Removes your gun
cg_fov - Zoom with any gun