Press the ` button to bring down the console. Then type one of the following. Make sure you press enter after every cheat and when you've finished typing the cheats press ` again.

God Mode-please god
Invisibility-please invisible
Kill all enemies-please killall
Give all weapons-please giveall
Open all doors-please open
Fly mode-please fly
No clipping-please ghost
Resets health to 100-please refresh
Show all messages from Netrisca-please tellall