Just wanted to second the recommendation to build something that already exists first. Like how artists do master studies, or still life, software developers can learn a lot by actually building something that already exists. It helps educate you about the nature of how certain things are built, and what considerations they have.

My first web project in Go was an email signup server. The second was a blog engine that still powers https://junglecoder.com. Both projects taught me a lot, and the blog took quite a while to finish up.

Getting something out the door and in front of other people, even if it's just your friends, is key.