check the voltage whilst cranking the car (at the battery) if it drops to around 6v the battery is flat.
start the car, jump it if you have to! now read the voltage at ther terminals of the alternator, you should get at least 13v, if not it is goosed. if it does read 13 then i would say your battery has failed.
when you remove the alternator, check the condition of the brushes (there is a small plastic part, held by 2 screws, this is the voltage regulator/ brush pack. these can be replace if they are worn) also make sure the small wire connected to the capacitor on the side has a good connection, on my gt the wire had corroded to nothing, which caused the alternator to fail.