You start by contributing to open-source projects. Minor fixes are ok but, I really encourage young people to really get involved into the project to the point they are considered maintainers. It will not only teach you how to work in a professional environment but, in most cases, it will teach you things that many companies get wrong (proper use of git and code-review to name one).
For sure, I am working on a personal project right now that is still in development. Heres the repo of it if anyone is interested github.com/Squwid/bytegolf
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