You must follow the following directions to get these cheats to work
1. Start Half-life 2 activate the console option start the game and press "~" to bring up the console.

2. Enter The Command "sv_cheats 1" In The Console. Cheats Are Now Enabled.

3. Here's A Partial List Of Cheats.

/impulse 101 - All weapons and ammo
/God - Godmode (Invincibility).
/God 0 - Godmode off
/Noclip - No Clipping (Walk Through Walls, Etc.)
/notarget - Opponents do not shoot you