These cheats won't work unless you begin the game by clicking 'hl.exe-console'. You can do this by adding '-console' to the end of the Half-Life shortcut properties. Note that the '/GIVE' commands only work if you start the game with the extra '-dev-console' parameter. The god mode and the map cheats only work with the '-console' parameter. When you've done this you can access the console using the ~ key (surprise, surprise), and for the final cheat initialisation first type 'sv_cheats 1' into the console.
/GOD god mode
/NOCLIP walk through walls
/MAP xxxx go to level xxxx as follows (single-player):