Note: Always backup your files before altering them! First, navigate to [game folder]\Data\Scripts and open the init.cfg file in a text editing program.

Find the line:

d.consoleh = 0

And change it to: d.consoleh = 1

Save the file and, while playing, press "~" or the key above Tab to bring up the console. Enter the following codes for the desired effect:
pl.health = X - Add Health (X = Amount of Health)
d.fly=1 - Fly
giveall - All Weapons
godmode =1 - God Mode
d.noclip=1 - No clipping (walk through walls)
unlimited_ammo =1 - Unlimited Ammo