Start with children's books. My mom has always done this, and she taught it to me. She would check out a stack of kids' books on a topic and read through them for a complete overview: the basic terms, concepts, events, and people, etc. It is a way to dive into a new subject in a completely non-intimidating way, and you really pick up a lot of knowledge.
Learn to make connections. Your brain does this naturally, but it makes better connections as your knowledge grows and you have more practice. I recommend learning to
MindMap/IdeaMap. That way you can graphically record the topics and ideas you're learning that go together, having your ideas handy to review later.
Also, making these maps will lead you to make connections you didn't think of at first, which is one of my favorite things about this. I've had so many "aha! moments" doing this.
Write it Out. (Or talk it out, if you're an extravert with a patient audience!) Although I disliked it for the first few weeks of ninth grade, I got used to it pretty quickly: after my daily Great Books reading session (about 45 minutes of reading each assigned title) I wrote a 10-minute narration of what I had read that day, and all my thoughts about it. It really helped me grow as a reader
and as a thinker.
“If we are honest, we can still love what we are, we can find all the good there is to find, and we may find ways to enhance that good, and to find a new kind of living world which is appropriate for our time.” ― Christopher Alexander