HTML5 (Javascript), Python, C#
I'd include all of them on an eventual "must learn" list.
In that regard it is difficult to go wrong with any of them. None of them are bad.
Of those, Python is likely the most immediately useful with long-term value. It is convenient to pop open a python window, type in some commands, and get an immediate result. If all you know is C++ you may be surprised at just how quickly you can accomplish many tasks.
After that, HTML generally is something everybody expects of developers. JavaScript less so, but if you're a developer and you cannot read the HTML source your geek credentials will be revoked and other developers will openly mock you when they discover it. Not so much at your age, but in five years or so you'll need it. You will need to learn it eventually.
The transition to JavaScript directly might be a bit tough if you haven't mastered the basics of HTML markup. You'll need to to be comfortable with the DOM (Document Object Model) used behind the scenes on HTML and related markup languages.
The transition to C# would be easy but not very educational. If you are shaky on the basics and not comfortable with a big step, C# might be good to help strengthen some of those fundamentals.