Thanks for reaching out!
There are many ways to gain experience working on bikes, from working on your own bike at home to an entry level position at a bike shop (typically checking in bikes for service and/or assembling new bikes, as an example). At REI, our bike shop master technicians and certified technicians are trained at Barnett's Bicycle Institute. To learn more about opportunities to become a bike technician at REI, we encourage you to reach out to your local REI store and speak with a bike tech or a manager.
One of our superusers, @TomV, has tremendous knowledge of bike maintenance and works as a tech in an IBD. He may be able to lend some advice from his experience.
Hopefully this helps!
@REI-JohnJ has already mentioned Barnett's which is pretty much considered an industry standard to measure against, and works more or less like a certification (like how a computer programmer can get certs in certain coding languages like C# or Python, CCNA, etc.). Like most certifications, it is a paid course. An employer may cover this cost, but if you're taking it on your own before landing a job it's coming out of your own pocket.
That said, Barnett's is by no means required at entry level: I have not taken it myself. My experience is mostly from watching all of Park Tool's repair help YouTube series starting about 2 years ago, or reading from sheldonbrown.com (ancient web graphic design aside...), and working on my own bikes at home. These excel at the process of doing a repair, but something I've found that really only comes from experience is diagnosing when you need to perform the repair in the first place. I started working at my shop about 3 months ago at entry level, so I'm still quite green professionally, but I can handle most of the bikes that come through our doors. I still have plenty to learn when it comes to hydraulic bleeds and suspension service. But the most important thing to landing the job was just putting myself out there and applying, a solid grasp of basic mechanical concepts like leverage and torque, and a willingness to learn.