How to find out programming

I have a 1999 Chevy Sub. that I can't get a new key fob to program. I have tried the 4 - 8 jumper and the ON, OFF, ON , OFF. How can I find out what needs to be done?? I bought this used, it had one key fob with it, but I would like to add another. It does have the rear window that I would also like to have control over. The fob that I received has LOCK, UNLOCK, a Red PANIC and a GM button. When I push the LOCK or UNLOCK the horn sounds. But if I push the GM button and then the LOCK or UNLOCK, no horn is heard. Do I need to find the main unit to see what the numbers are?? Please help!!

Thanks, David