First, modify the shortcut to the game's executable by adding the switch:

+ set sv_cheats 1

to the command line. You can also run the executable via MSDOS and manually type in the switch each time. That done, during the game, press the tilde key (~) and you can will be allowed to enter the following cheats:

/god
/notarget
/noclip
/giveall
/give luger
/give colt
/give dual colts
/give mp40
/give thompson
/give sten
/give mauser rifle
/give sniper scope
/give snooper rifle
/give fg42 paratroop rifle
/give grenade
/give grenades
/give pineapple
/give pineapples
/give panzerfaust
/give venom
/give flamethrower
/give tesla gun
/give 9mm
/give 9mm rounds
/give 9mm box
/give 7.92mm
/give 7.92mm rounds
/give .45 cal
/give .45 cal box
/give .30 cal
/give .30 cal box
/give 12.7mm
/give fuel
/give cell
/give armor ###
/give health ###
/give binoculars