First of all,you will need the Northstar waterpump removal tool, You can get this from most reputable auto part stores and looks like this :-
The following link shows a selection of different manufacturers.
You need to remove the air filter housing and ducting to gain easy access to the pump.
Remove the shield covering the pulley and belt. The belt is easily removed by levering the tensioner pulley.
The water pump is located at the lower radiator hose manifold where you will also find the thermostat.Remove the manifold cover and you will see the water pump.
Insert the removal tool over the pump making sure it is locked in position.You will need to use a breaker bar on the removal tool and turn the tool CLOCKWISE to release the pump (1/4 turn), or if you imagine the breaker bar being vertical, push towards the windscreen of your car,it takes quite a bit of force to unseat the pump hence the use of a breaker bar.
As for the waterpump itself, DO NOT buy anything other than AC DELCO as they are the highest quality and is a genuine GM replacement part.
Water pump part number AC Delco 252707
Installing the new pump does not require as much torque as removing the old one.
Tighten the new pump to 73 ft.lbs. making sure it is seated properly before tightening.
Don't forget to clean out the water pump housing prior to installing the new pump and don't forget to include a new water pump gasket and housing cover gasket.
It is also a good idea to install a new thermostat while you're at it.