Go to RUN on your start menu. Type in - HL.EXE-CONSOLE
You will have to start the game like this everytime you
want to cheat. The only thing different will be a console
button in the menu screen.
Click on it then type in - SV_CHEATS 1 - this will
hit ESC to go back to the menu screen. Load you game now.
When your game is started stop in a safe place and hit the
Tindle key ~ on your keyboard. (top left under Esc button)
When the screen pops down keyin - SV_CHEATS 1 - enter.
Then keyin /god for god mode or /noclip to fly through walls
and stuff or keyin /give_ammo buckshot or replace buckshot
with the following if you want other stuf - mp5clip,
gaussclip,egonclip (egonclip smetimes crashes the game)