Select Options, Keyboard/Mouse, then Allow Developers Console. Press the ~ button during the game to access the console and enter "Sv_Cheats 1" to enable cheats. Then enter the following for the noted benefit:

give ammo - Full Ammunition
give pistol - Get Pistol
z_spawn weapon_SMG - Gives you the SMG.
god - Invincibility
noclip - Toggles No Clipping Mode
sv_infinite_ammo 1 - Unlimited Ammunition