Create a shortcut of HL.EXE, then add -dev -toconsole to the command line. Start Half-Life with this shortcut. (This shortcut is added to make the pop-up console "work." 

NOTE: to enter the console, press the ~ key.

Start the game with "map c0a0" to start a game as you would start it normally.  But if you want, load a auto-saved game to start where you left off hit F7.

Use these cheats during gameplay:

god = Invulnerability
fly = gravity 0 with full control
impulse101 = all weapons, ammo and items
noclip = fly through any object you wish
notarget = enemies do not react to you unless you inflict damage

/give object_name = where object_name is a specific item, it gives you the item.
For example: /give item_healthkit would give you a health kit.
Here are some other items:

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