The radiator drain plug/valve is located on the driver's side of the radiator towards the engine. To access it you will have to jack up the front driver's side of car, then remove the plastic splash guard and/or air deflector from underneath the car. Several 10mm bolts may secure the plastic. Once that is removed all you need is a tool like a 1/4" drive socket handle or ratchet. Insert end of tool into square cutout in plug and turn about 1/8" - 1/4" counterclockwise. Coolant will drain.
Note: The coolant may be hot so follow proper safety procedures, let engine cool down and when jacking car up be sure to use jackstands.
The pics below show the tool I use, the drain plug and hopefully should be self explanatory.
If not feel free to ask questions in this thread.
Applicable to '97 and up Buick Regal and Century and '97 - '03 Pontiac Grand Prix
1/4" drive tool and drain plug removed from radiator for illustration purposes
Coolant being drained
Location of drain plug on removed radiator