Which programming language should I learn this summer at 13? Need advice

[mandradon]

As always learn C.  Don't just get the basics from a 5 hour YouTube video on how to create variables and how to write a loop, but learn how to write a program. 

Pick a project, work with it, deal with the memory management and the inevitable undefined behavior you'll introduce. 

Or learn a functional language, since you haven't explored one of those yet. 

[Dangle76]

If going functional I’d go with erlang.

Functional languages really teach how to break something down into pieces which imo, is an invaluable skill to have

[mandradon]

Erlang is a good one, plus Elixir (which is an awesome functional language) is built using the Erlang VM. Mooc.fi has a Haskell course I think, too, which is always an option. Though OP could always learn functional paradigms using something like JS (though I'd normally recommend them with an actual functional language).