Start Half-Life with the console. To do so edit the shortcut to hl.exe. Add -console.
It should look something like: "hl.exe -console"

The /GIVE command works if you start the game with the -dev -console parameter, and that the God mode cheat and map change cheat only work with the -console parameter.

This will let you access the console using "~" key.

You may have to type "sv_cheats 1" in console first.
The following commands are available in console:

IMPULSE 101 - Give all weapons and ammo.

/GOD - god mode.

/NOCLIP - walk through walls/fly.

/MAP xxxx - go to map xxxx. Here's the maplist:

Single Player:

c0a0 ; c0a0a ; c0a0b ; c0a0c ; c0a0d
c0a0e ; c1a0 ; c1a0a ; c1a0b ; c1a0c
c1a0d ; c1a0e ; c1a1 ; c1a1a ; c1a1b
c1a1c ; c1a1d ; c1a1f ; c1a2 ; c1a2a
c1a2b ; c1a2c ; c1a2d ; c1a3 ; c1a3a
c1a3b ; c1a3c ; c1a3d ; c1a4 ; c1a4b
c1a4d ; c1a4e ; c1a4f ; c1a4g ; c1a4i
c1a4j ; c1a4k ; c2a1 ; c2a1a ; c2a1b
c2a2 ; c2a2a ; c2a2b1 ; c2a2b2 ; c2a2c
c2a2d ; c2a2e ; c2a2f ; c2a2g ; c2a2h
c2a3 ; c2a3a ; c2a3b ; c2a3c ; c2a3d
c2a3e ; c2a4 ; c2a4a ; c2a4b ; c2a4c
c2a4d ; c2a4e ; c2a4f ; c2a4g ; c2a5
c2a5a ; c2a5b ; c2a5c ; c2a5d ; c2a5e
c2a5f ; c2a5g ; c2a5w ; c2a5x ; c3a1
c3a1a ; c3a1b ; c3a2 ; c3a2a ; c3a2b
c3a2c ; c3a2d ; c3a2e ; c3a2f ; c4a1
c4a1a ; c4a1b ; c4a1c ; c4a1d ; c4a1e
c4a1f ; c4a2 ; c4a2a ; c4a2b ; c4a3
c5a1

/GIVE xxxx - give item xxxx. The following items are available:

item_airtank ; item_antidote ; item_battery
item_healthkit ; item_longjump ; item_security
item_sodacan ; item_suit

ammo_357 ; ammo_9mmAR ; ammo_9mmbox
ammo_9mmclip ; ammo_ARgrenades ; ammo_buckshot
ammo_crossbow ; ammo_gaussclip ; ammo_rpgclip
ammo_glockclip ; ammo_mp5clip ; ammo_mp5grenades

weapon_357 ; weapon_9mmAR ; weapon_9mmhandgun
weapon_crossbow ; weapon_crowbar ; weapon_egon
weapon_gauss ; weapon_glock ; weapon_handgrenade
weapon_hornetgun ; weapon_mp5 ; weapon_python
weapon_rpg ; weapon_satchel ; weapon_shotgun
weapon_snark ; weapon_tripmine ;