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

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