Learn a functional programming language. This has had the most impact on the way I think about systems by far. The biggest impact was realizing all of the time I've spent trying to master all of the OOP acronyms hasn't paid off. I've also realized that OOP might be meant for a type of programming that I don't do (video game logic? device drivers?), where as FP seems to fit great for Web applications that are just wrapping databases and spitting out HTML or JSON.
As for FP languages for a starter, I'd recommend any of Erlang/Elixir, Clojure or Scala (only for its FP features and great learning resources)
As for FP languages for a starter, I'd recommend any of Erlang/Elixir, Clojure or Scala (only for its FP features and great learning resources)