During gameplay press TAB, after type one of the following codes:

Effect - Code

God Mode - God
All Weapons - AllWeapons
Max Ammo - AllAmmo
Max Ammo and All Weapons - Loaded
Flight Mode - Fly
No Clipping - Ghost
Infinite Oxygen - Amphibious
Teleport to Crosshairs - Teleport
Third Person View - ViewSelf
Invisible to Enemies - Invisible True