Start game as usual. During gameplay press tilde to go to console, then enter the following cheats:
ADDCASH - Get Some Cash
NEXTMISSION - Complete Mission
FAILMISSION - Fail Mission
ROCK - Enable God Mode
GOD - Enable God Mode
GIMMEALLMISSIONS - Enable Cruise Mode
FREECAM - Enable Fly Mode
JUMP - Press Left Shift To Jump
SPEED - Press Left Shift For Turbo Boost
TELEPORT - Press R to Teleport To Random Locations
CAMERAEDITOR - Show Cameras
CARCAMEDIT - Move Camera
SETLIGHT - Show Light Settings
SHOWNODES - Show Nodes
SHOWSOUNDS - Show Sound-Lines
EXIT - Exit Game