To use the following cheats you must first enable the console. To do this go to the Audio/Video menu, then go to advanced options. Click console on. Now during the game you can type any of the following codes:

give all = Gives the player all weapons
god = Activates god mode
map x = Level select, X = name of the map
noclip = Deactivates clipping mode
health 100 = Grants the player full health.