My 10-year-old son brought home a book from our park’s free library box. It was a biology textbook – teachers edition. He said it looked interesting and hey, it was free (having no idea you could sell it). I scanned it in my Amazon seller app and realized it was worth around $150. He was so excited. We listed it for sale for $130 and it sold! Going to tell him, he just made $130!
Focus on what you're good at. What interests you, what do you enjoy? If there's something you've always loved that has any possibility of a commercial link (and in this day and age, even water has been commercialized), then build upon that and help others to find your info/webpage/blog/ebook. You really can do anything, but you can't just blag it, you have to stay true and honest to what is possible for yourself and your enthusiasm will only come across if you're genuinely into that interest.
How much will you make? I think my best month with Google AdSense was almost $5,000 over the last ten years. That amazing month blew my mind since it was actually near the beginning of my blogging journey. When you go from making zero to $5,000 in a month, that will rock your world. For me, it also got me even more excited because I knew there were other ways to monetize.