First you'll need to bring up the console by hitting the tilde key (~) or by hitting Escape, selecting Options and then Console. Then you'll want to type in one of the following to get the cheat you want. I have managed to get most of these commands to work.

GOD	god mode
MAP E1M?	warp to ? map
NOTARGET	enemies don't see you
NOCLIP	walk through walls
IMPULSE 255	quad damage mode
FLY	you can fly (D for up, C for down)
GIVE ?	weapon (enter a number for ?)
GIVE S ?	shotgun shells (replace ? with 255 or lower)
GIVE C ?	battery (replace ? with 255 or lower)
GIVE R ?	rockets (replace ? with 255 or lower)
GIVE N ?	nails (replace ? with 255 or lower)
GIVE HEALTH ?	health (replace ? with 999 or lower)
IMPULSE -1	quad damage mode
IMPULSE 1	shotgun
IMPULSE 2	supershotgun
IMPULSE 3	nailgun
IMPULSE 4	chainnailgun
IMPULSE 5	grenade launcher
IMPULSE 6	rocket launcher
IMPULSE 9	all weapons
IMPULSE 11	jewels (do it 4 times for all)
SV_GRAVITY -50	gravity (replace with any number)