I'm not Wolf, and I didn't learn Ruby as my first language; but my journey was: 1. read a book that describes how the language works, and 2. try to use what I've learned to change existing code.
In your case I'd recommend checking out a Ruby tutorial and trying to add simple-ish new things (e.g. moves/abilities/items, rearranging the UI) to Essentials, and move on to more complicated things (e.g. adding new options to menus, creating some of the things Wolf has done in the overworld) once you're comfortable.
EDIT: The most important thing is to practice lots. If you need inspiration look at a fan game/official game that has a feature missing from Essentials and try to add it (e.g. Reborn's field system, or the missing gen 7 abilities); or try to rewrite an existing feature without looking at the code. I'd recommend trying to help people write code, either one-off questions here on the forums, or joining a team that's looking for a coder, or perhaps offer to work on (broken link removed).