Hi! Sure, I can answer this! I've been a professional developer for almost 20 years (I got my first job around December 1999). I first learned about open source because I wanted to learn to program and we couldn't afford to pay for commercial compilers like Borland's C compiler. I think this experience is what really inspired me to be part of the Open Source community. I love programming and would like it if all people had the opportunity to learn to code.
As for getting a job at GitHub, I can't say it was easy. I was programming for 16 years before starting at GitHub, so I had a lot of experience. I applied for a job, and fortunately I was qualified enough to do it!
My best advice for becoming a better developer is to practice. Don't be afraid to make mistakes, just try something and see if it works out. The most important thing to do is look back on the things you've tried and figure out what worked and what didn't. I think those experiences and self reflection make the best developers.