I would start with JavaScript.
A very useful language that you can do a lot with, and see results quickly. It can stretch from very basic to very sophisticated (simple web pages to server side functionality like Node.js).
But if you want to program your Arduino, learning basic C++ would be a good start.
Any language you learn changes your next question from "how do I do this?" To "how do I do this in python/java/ruby/swift/C#/Perl/.......?"