Resources To Consider

• Wikipedia. I used Wikipedia to put together a lot of the start of this mind map.
• YouTube. Don't discount all YouTube videos as not being credible, there are many awesome people putting out great content for free on YouTube.
• Communities - online or in real life. Having access to other people interested in a topic can help immensely. Some online communities can be found on Reddit and Discord, and you might have some luck in real life on Meet Up.
• Books and podcasts.
• Online courses. Many of these are free or incredibly cheap, and are even offered by reputable institutes around the world. Consider Udemy, Coursera, EdX and Khan Academy.
• Do a good old fashioned Google search. You never know what you'll find.
• Compile your own resources as you go.

Study Aids

• Flashcards to help with memory recall. These can be used for an incredibly diverse range of topics.
• The Feynman technique can help you understand complex concepts.
• Online quizzes and tests on the subject matter.
• Practical applications of your knowledge (where applicable).