How do I learn an new domain from scratch?

I am looking to start studying a new domain as a complete novice. In this case I am learning about the energy sector and innovation, in both the public and private sector (and the overlaps).

What are the best ways to: -Learn a new domain from the ground up -Keep track of what I've learned -Write about the topic as I learn

I have tried Evernote and some of the academic PDF organizers from my time in school. I've heard good things about physical flashcards or programs that link ideas to created connected graphs.

Ideally, I would use the tool with the least friction and spend my mental energy on learning the subject matter.