Head to the "Options" menu, then select "Keyboard" and "Advanced," where you can check a box to enable the cheats console. Now press ~ (TILDE, THE KEY ABOVE TAB) to prompt the console to appear, and type in SV_CHEATS 1 to enable it. Press ENTER to confirm. Now you can enter any available cheat codes, as indicated below.

Code - Effect
IMPULSE 101 - All Weapons and Some Health