How do you get your air brake license?

For driving an air brake equipped vehicle, or for working on them?

To work on them, you have to get the ASE certification. You must take and complete that ASE test to become certified.

To drive a vehicle equipped with them, assuming you're in the US, there is no such license. If you have a CDL, then you must complete and pass the written test and do your road test in an air brake equipped vehicle, or else you get a restriction which prohibits you from operating a commercial vehicle equipped with air brakes.