It heavily depends on what you want to do.
No matter what, I would recommend:
- Python is pretty easy to get into and useful for a lot of general purpose tasks, parsing text, automating tasks, doing math with numpy, and plenty of other stuff, so you should learn it. Its main problem is that it has a slow runtime.
- C is ubiquitous and most of the software we use from day to day are C or C++ under the hood. It has a relatively simple design – though it is hard to use it correctly –, has libraries for basically everything, is very fast, and you will gain an understanding of how memory management works by learning it, which will make you a better programmer in the long run. Also, most languages have a C FFI.
- Getting familiar with Unix shell scripting, pipes, file redirection, and tools like grep, find, sed, awk is highly recommended. It's a massive gain in productivity as it can allow you to do pretty complex text/file manipulation by writing very little code in some situations.
All of these are installed by default on most Linux distributions.
Other than that, what do you want to do?
- If you want to do Android development or get a dull codemonkey job, learn Java.
- If you gotta go fast and using C is getting too painful, learn C++ or Rust.
- If you want to fuck around and learn CS concepts, learn a Lisp dialect or Haskell.
Your choice of language will mostly be determined by the availability of libraries and reusable code related to your problem space, and the trade-offs you are willing to make between convenience and speed.
Most mainstream languages are pretty similar, so you can easily pick one once you are comfortable with paradigms like plain procedural programming, object-oriented programming and functional programming. Most experienced programmers are more or less polyglots for this reason. Once you will be pretty comfortable with basic programming and want to get more advanced, you should learn all these different paradigms, as well as standard algorithms and data structures (C is pretty good for learning the latter).