The simpler route, by far, is to buy a custom calibration that is already programmed for your car, whether modified or stock. We can highly recommend these very experienced sources for GM cars:

http://www.pcmforless.com/
http://www.madz28.com/
http://www.tbichips.com/

Doing your own custom calibration takes more hardware and software, and a lot more knowledge. Good sites to get smart on chip upgrades for GM cars are at:

http://www.carputing.com/

Be aware that you can seriously damage your car by loading the wrong parameters into your ECU/PCM. Study up on chip upgrades very carefully before even beginning to consider doing it for yourself.