Cheat Mode:
Start the game with the +set console 1 command line parameter. Press ~ to display the console window, then type cheats on to enable cheat mode. Then, enter one of the following codes in the console window.
*Note: Check below for proper procedure.

Proper Procedure:
Create a Shortcut to the game on your Desktop. Right click on it, go to properties. You will see something like: "C:\Games\****\****.exe"

Now type +set console after it: "C:\Games\****\****.exe" +set console 1

Code                    Effect
cheats 0                Disable cheats   
screenshot              Screenshot   
god                     God mode 
health (100 or greater) Additional health   
weapon_give_(1-10)      Weapon select   
massacre                Kill all monsters in current level   
notarget                Invisibility  
noclip                  No clipping mode   
cam_toggle              Cycle three camera views  
cam_nextmon             Camera focused on next monster   
cam_nextmon             Cycle all characters on level   
cam_nextsidekic         Cycle sidekicks on level k  
timescale (speed)       Change game speed, 1.0 is default  
map (level name)        Level select   
bind (key and command)  Key command binding    
connect (server)        Select active server    
developer (1 or 0)      Toggle game engine messages   
flushmap (1 or 0)       Toggle clean map load with no caching   
r_speeds (1 or 0)       Toggle rendering speed display    
r_drawflat (1 or 0)     Toggle flat-shaded polygons   
r_fullbright (1 or 0)   Toggle lightmaps on surfaces   
gl_polylines (1 or 0) 
(server)                Toggle line drawing (not available on Voodoo cards)