Honestly, Discourse is a pretty complex app – more complex than a few written tutorials can teach. I highly recommend getting a solid base of understanding on Rails and Ember.js, then digging through the codebase, tracing codepaths as best as you can (though I will say this is not easy at first). Also be sure to look at plugins to see how they tie into Discourse (as there are good ways and bad ways to do this).
@angus wrote up a fantastic beginners guide on the basic knowledge you’ll need to get started.
Of course, a lot of this is going to seem like a foreign language until you start to build and experiment further. I’ve been developing on Discourse for 1.5 years now and am only now feeling like I have any sort of understanding how the app works.