Enable the console window in the game options. Then, press ~ (or `) during game play and enter one of the following codes at the console window to activate the corresponding cheat function.

Effect - Code

Get all weapons - wuss
Toggle god mode - god
Toggle no clip - noclip
All weapons - give all
Set health level - health <1-100>
First person view - cg_cameradist -45
Return to default view - cg_cameradist 128
Toggle frame rate display - fps <0 or 1>
Disable enemy AI - notarget