Open the console during a single player game by pressing CTRL, ALT and ~ at the same time. Then type the following command:

net_allowcheats 1

Afterwards, you are free to input any of the following cheats for desired effects. Note that those cheats only work in single player.

Code	Effect
com_allowconsole 0	Disable Easy Console access
Com_allowconsole 1	Enable Easy console access
Give ammo	Gives you more ammo
Give health	Gives you more health
ammo	Gives you some ammo
god	Infinite health, you have to type this everytime you respawn, re-enter to disable
noclip	Makes you go through walls, re-enter to disable
g_maxproficiency 1	Promotes you to maximum rank and unlocks everything, enter g_maxproficiency 0 to disable
pm_thirdperson 1	Third person view, enter pm_thirdperson 0 to disable