During gameplay press ~ to get the console then put in a code below

give all - Get all items
give life - Get Life
pause - Pauses game toggle
g_p_god - Invincibility
give jetpack - Get jetpack
give ammo - Get ammo
give nuke - 10 Nukes
give secret - Secret marked as found
give keys - Get keys
Give forcefield - Get forcefield
toggle r_stats - ????
camera player - ????
kill - ????
toggle r_timings - ????
toggle g_map_info - ????
toggle g_debug - ????
camera camera - ????

Other Cheat Method:

Press ~ to bring up the console then type in exec
cheats.cfg and press ENTER to activate cheats

Now press a letter below

G - Get Everything
H - Max Ammo
J - Get Jetpack
F - Get Forcefield
L - 1 Extra Life
K - Self-Destruct
I - Invinicibility
P - Pause