Come up with a project and write code. For example you can just implement the web server you were mentioning. That is a good start. Build it and see how to scal

Come up with a project and write code. For example you can just implement the web server you were mentioning. That is a good start. Build it and see how to scale it to handle millions of requests. Just creating the code to test it will be a good exercise. You can dig into a lot writing a web server. After that you can pick up another project in the kernel.