The
Getting Started tutorial teaches the fundamentals of how to make changes to the game, but the rest of the guide is more of a reference than guided lessons. Most of what I know is documented there, though.
I'd suggest poking around the files and seeing what you can change without worrying about making anything "fancy," and just try to get a feel for how parts of the engine fit together first. The guide can help demystify stuff as you go along, and if you ever get lost there are several of us here or on the discord who can help point you in the right direction.