Use Reddit. You don't need an existing audience for that. Make sure to follow the rules of the subreddit. Write a few tips/tutorial posts, help other learners, etc. Then you can try self promoting within the rules. And parallelly, you can build an audience on sites on Twitter. Read about marketing. Follow other creators. And so on. (Edit: You can self promote without existing audience with Show HN too, provided it fits the rules. Luck plays a huge role, but you won't get lucky if you don't try at all).

Regarding RoI, that depends on your needs. I left my job in 2014 due to various reasons. I tried and failed at many things for about 5 years. Then I tried selling ebooks and the second book clicked for me. Now I earn more than I need (with the caveat that I live alone in outskirts of a second tier city with an average monthly expense of just about $150).