Factory radio to after market

The easiest way is to get a plug in conversion harness. Then you don't need to cut and splice the factory harness so you don't end up accidentally connecting a wire in the wrong place and burning out anything like the body control module. Be advised you may lose the chimes and warnings with the new radio because those are on the network that connects to the OEM unit and will not be on the aftermarket unit. You can buy a wiring adapter that will retain them if you wish. If you have the optional On-Star system you will also lose it's use as it runs through the OEM radio and will not interface with an aftermarket unit. If you also have the factory amplifier option you will need either a line level voltage converter or to replace the speakers as the OEM amp will not accept speaker level input signals.
For the wiring as well as information on which options are available you may want to look at Crutchfields web site. They will give you a list of items to make the process easier.