Find an existing open source project and fix some bugs. That will give you experience in larger projects, communication, reading documentation, understanding frameworks, using source control, debugging, reading code, and possibly in dark corners of your chosen language.

Then do it again, and again. You will earn respect in the community, and experience that is otherwise hard to obtain.

You can do that quickly, too. It's the fastest way to learn, and you'll be well placed to take that one burning idea that you can't ignore and make it a reality.

If you want to make a mirror for a 6" reflecting telecope, conventional wisdom and experience says it's fastest first to make a 4", then the 6". The experience gained in the smaller exercise far outweighs the time taken.

It's an analogy, and it has flaws, but it contains a lesson.