How is what done? How does the light turn on or how is the system serviced?

The Air Bag Computer runs some self-tests and when it detects a problem it turns on the warning light, sets a diagnostic fault code in memory, and disables the system. Having the fault code(s) read is the place to start. That will get you to the circuit or system that needs further diagnosis.

Your first step is have the air bag computer scanned for codes and left me know the exact code numbers you have.Most diyer scan tools wont read air bag codes.