Create a shortcut and edit the properties of it and add this after all the other stuff: "set monkeytoy 0 set svcheats 1" without the quotes
Start the game and press the tilde sign to open the console.
Note: If after you type one of these codes in the console it tells you that "svcheats are read only", then try typing this into the console: setrandom svcheats 1
Enter some of these commands:
give - Gives player an object "give mp 40", etc.
give health - gives you full health
god - Invulnerability
notarget - The enemies won't shoot you
noclip - Walk through walls and such.