How do you enable the cheats in half-life 2

enable the console, then go to it and type "sv_cheats 1" without the quotes. From here, use codes like "god", "impulse 101", "noclip", and "physcannon_mega_enabled 1" to have some real fun with the game.
Note: sv_cheats will turn off achievements for the game, so it's good to find some commands that don't take sv_cheats. Once you've enabled them, the achievements are off for the rest of the game, even if you turn off sv_cheats (sv_cheats 0)