I'm assuming you have the stock Motorola alternator (might want to fill out your profile).
If you had the alternator checked and it isn't putting out any voltage, then my guess is it is bad. If you can get it working and the system won't charge, the next step is to have the voltage regulator checked.
After that, you can check to see if you have power to the alternator warning light (if the light bulb is bad, the charging system won't work) and then on to the alternator itself.
In the end, an upgrade to a GM internally regulated alternator can be done with a minimum of effort.