Maybe you should take a break from SerenityOS and write a book about the useful/practical subset of the C++ language??
Seriously, you don't even have to know programming to contribute. Writing documentation, porting software, trying to boot on real hardware, trying out websites with the web browser, looking for bugs in the user interface... There's no such thing as a barrier to entry with SerenityOS as long as you're motivated to learn.
No, kidding... But their Jakt[1] language will make it much easier to contribute than in arcane reality of C++.
Can just open any app and you will see little things that would make for a good first PR. The codebase is also full of Fixmes, many of which are very approachable for a beginner.