To use codes, you have to run the game with the console
enabled. Launch Half-Life in developer mode by going to
your Windows Start menu, selecting "Run", and typing 
C:SierraHalf-Lifehl.exe -dev -toconsole
Now load the last quick saved game (F6), then press 
the ~ key to pull down the console. Type in sv_cheats 1
and press Enter. Cheats are now enabled, but you need
to die or reload your game before you can use them.

Now during play, press the ~ key and type in the 
following codes:

/god            - God mode on/off
/notarget       - Enemies ignore you
/impulse 101    - All weapons and ammo
/noclip         - No Clipping (Walk Through Walls...)
/give XXXX      - Gives any item. In place of "XXXX", 
                  enter one of the following:

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_egonclip 
ammo_gaussclip 
ammo_glockclip 
ammo_mp5clip 
ammo_mp5grenades 
ammo_rpgclip
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