Enable the console from the game's in-game options. Hit the tilde key (~) to drop the dev-console. Type seta thereisacow "1337" and then press ENTER. Type spdevmap bog_a and then press ENTER. This will start the demo with cheats enabled. When playing, drop down the dev-console and enter the following codes and hit ENTER to activate or toggle them.

give all: Access All Weapons
god: God Mode
noclip: No Clipping Mode
notarget: Enemies Ignore You
give ammo: Full Ammo
jump_height #: Adjust Gravity (Default = 39)
timescale #: Adjust Time (Default = 1.00)
cg_LaserForceOn 1: Add Laser Sight to All Weapons