During gameplay press the ~ key to open the console. Type iamtheone and press enter. You may use any of the cheats below:

god = Invincibility.
fly = You can fly around.
walk = Turns off ghost and fly.
ghost = Turns off clipping.
loaded = Gives all weapons.
allammo = Full ammo for weapons currently in your inventory.
playersonly = Freezetime.
behindview 0 = Default view.
behindview 1 = Thir-person view.
open (map name) = Skip to map.
summon (item name) = Give item