I started when I stumbled over the Atom editor project, about what... 2 years ago? This was just before they first open-sourced it (Electron was still a neat little sub-project of Atom called "atom-shell").
I loved the idea of using web tech (since I had just started learning Javascript) and how vibrant and cooperative the budding community was at the time. So I started engaging on their forum, and familiarizing myself with Github. I helped with issue triage, solving newbie problems etc. Eventually, I also contributed to Pull Requests and opened one or two of my own, which was amazing:
It taught me about best practices on Open Sources projects, Git and Github-based development workflows, how to cooperate with people across the globe etc.
As I'm not a full-time professional developer, the community eventually grew over my head when I couldn't spend much time there anymore. But Atom is still my editor for everything and I have a lot of love for it mostly because of the Open Source experience it gave me.