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.

1. These are definitely the 2 most common, but anywhere you can use an affiliate link, you can make money. I see a lot of people spamming Quora with affiliate links but wouldn’t advise it. People who will actually buy the product through your link are the ones you will educate on why they should buy it. You need your own platform somewhere and blogs/YouTube and usually best.
Once you’re financially stable, I hope you start giving back. It feels good and people like the idea of supporting a good cause (they will be more likely to click your affiliate link in your disclaimer). This also means you don’t have to use as many links in your content and risk getting a penalized. Last year I donated $3,000 to Red Cross At Hurricane Harvey.
Mechanical Turk: Amazon's Mechanical Turk is a resource for doing human-intelligence tasks, or as the site commonly refers to them, HITs. You get paid a very small fee for any given HIT and you'll need a good deal of volume to make a substantial amount of money. But it is a resource you can use in your spare time to generate a small income online. 
Once you have the affiliate links you need, it is time to present them to your audience. As you start using affiliate marketing to talk about products, you need to be aware of how your audience perceives you. If your website appears to be only after their money through an overwhelming amount of affiliate links, then they are more likely to simply leave your site.