Press ~ during game play to display the console window, then enter one of the following codes :
Result - Code:
Spawn indicated item and number - give (item name) [1-200]
All weapons - gimme
No clipping mode - noclip
Easy code activation:
After entering the cheat code and the item name, you can press [Cursor Up] to retrace the item name and get the ammo all over again, instead of typing everything. You must press [Cursor Up] repeatedly so that it can search for it.