If you’re planning on relying on SEO for traffic which most people do, it’s all about finding keywords where people would be interested in purchasing an affiliate product, and analyzing the competition of each of those keywords. Back when I was promoting Genesis themes, I saw hardly any articles about “Genesis eCommerce themes” (when I Googled it) which was a popular keyword. I got myself #1 for it. For SiteGround and StackPath, I saw opportunities to write articles on “settings” for each cache plugin (W3 Total Cache Settings, WP Fastest Cache Settings, WP Super Cache Settings, etc). After researching these keywords I was confident I could write better tutorials that the ones out there.
If you're serious about making money online, start a blog. Blogging is one of the easiest and most sustainable income sources. As long as the blog is setup the right way, in the right niche, with the right content targeted at the right audience, and the offer is complementary to the content, you could make a tremendous amount of passive income from a blog.