It is quite an involved process. I have apprenticed of 2 1/2 years and learned how to tune pretty well and have a way to go on regulation, rebuilding, repairs. I can perform most small in house jobs, sticking keys, loose tuning pins, maybe reglue a broken flange, splice strings, but that is a fraction of what there is to know.
What you need to do, is find a certified technician and ask if they take students. OR, register in a school that teaches piano technology. When you're ready, you can be tested to become a Regitered piano technican which is not mandatory, but its a good way to get your name out there. Its a mostly word of mouth business.
Start your research here:
https://www.ptg.orgUnfortunately, at my age with house and family to maintain, this is not my profession. I get a few jobs per month and really enjoy the craft, but its a little late to totally change my career.
Also, it takes most people YEARS to develop a clientele to make enough to make a good living at it. Most people buy a piano for the kid, they don't tune or regulate as often as they should, kid quits and the piano sits there and deteriorates and looks pretty with the pictures and candles on it and such.
Don't let me talk you out of trying. If you are a real go getter, you can do this. Just letting you know, don't quit your day job until......
You'll get out of it what you put in.