Start small. Answer a question on Stackoverflow. Or if you can’t, ask a question. Open issues on Github. Solve issues on Github. Create pull requests. Create your own open source project for selfish reasons on Github and maintain it. Rinse and repeat and don’t give up.

E.g. I created a scraper for a real estate website that found me the apartment I‘m living in now. Other people noticed that and started to use my software. In the end my software was mentioned in the local newspaper and the real estate website introduced captchas to prevent more scraping. The local real estate market became more fair, because scraping doesn’t work anymore. So I guess I changed the world to the better by my selfish actions.