What always caught my attention when giving through these projects was any project they had worked on. The actual project didn't matter so much. But almost always, how well they were able to understand what they worked on directly translated to their ability to present it in simple and concise few sentences.
Putting these pieces together - my advice is to go find something you want to build and build it. You'll figure out the best language, the libraries to use and all the good stuff on the context of figuring out how to build what you want to build.
The only real skills I strongly recommend you learn:
- git
- Makefile
- shell scripting
Bonus if you can run Linux on your primary computer. You'll learn a lot from just using a Linux box for daily use.
Go build something for fun. Write a text adventure game. Write a Sudoku solver. Write a reddit bot to count how many times someone said "wubalubadubdub" per day.
At some point, you'll have to transition from fun projects to projects that will look good on a resume. Keep that in mind and don't get lost in the woods and forget that you are on a mission.
Happy coding!