Frickin' build something, man. A JSON parser. A web server. A language implementation. Something. Anything. Read the article by that lady who advocated writing useless programs[0] for more ideas. Pick a language and just go. Avoid new and shiny; the idea here is to choose the language you're most comfortable in, so it's likely to be something you've
years of experience writing. Is that Java? Go nuts in Java.
The idea behind picking your coziest language is so that as you build the thing you're building, any snags you identify are more likely to be issues with you rather than the environment or your lack of proficiency in it. Do you get frustrated and procrastinate? Does your coding style make your code more needlessly complex or error-prone? As you work you will notice these issues. And, at a minimum, you will have a sense of where you can improve, so that the next thing you build, you can build faster, more easily, and more robustly.
I like working in Lisp because Lisp comes with frickin' productivity turbojets attached. No other language lets me turn my thoughts into code more quickly or easily. But even working in Lisp... I find that there's friction working on a complex or unfamiliar problem and my urge to quit, or at least put it off and play video games, starts to rise. I'm still working on fighting that.
[0] https://ntietz.com/blog/write-more-useless-software/