Welcome aboard! 
I’m also a beginner at Ruby, and I’ve found this tutorial really helpful to start off with.
Here’s a 7-part series on developing Discourse plugins, although it may not have exactly what you’re looking for, but it’s a good resource to help understand how a Discourse plugin is structured.
Personally, I learn best by example and logging each step of the way, so Rails.logger, along with Discourse’s logster, which you can access via localhost:3000/logs, are really helpful in doing that.
Here lies the Ruby documentation for Discourse core, if you are feeling brave and want to explore on your own.