First press the ~ key (just like in Quake and Q2). Then enter the code:

/health 999 - Health 999
/wuss - All weapons
/superfuzz - God mode
/nocollision - No clip mode
/wallflower - No target
/spawn magnum - Gives the Magnum
/spawn shotgun - Gives Shotgun
/spawn assaultrifle - Gives Assault Rifle
/spawn rocketlauncher - Gives Rocket Launcher
/spawn sniperrifle - Gives Sniper Rifle
/spawn heligun - Gives the Big Gun from helicopter

/spawn reactiveshields - Gives the Reactive Shield
/spawn rockets - Gives Rockets
/spawn coin - Give Coin
/spawn health - Gives a Healthpack
/spawn cookies - Gives a pack of cookies
/spawn lensflare - Makes a light effect