Well, it depends on what you are interested in? Me personally, I would earn money in any possible way online, I've done tons of things to earn money, arranging from posting on blogs and forums and creating blogs and forums with ads in it; creating YouTube videos with monetization and joining Amazon's affiliation program; and even by clicking and viewing ads and even writing in forums, you could get paid online.
Blogs
There are millions of ways to earn money online, the hardest and the most earning method to earn money for me is to create a blog. Blogs are a pretty hard thing to make, you need the talent to create one, involving knowledge, writing, and designing. So where do I get money from? From monetizing the blog with AdSense. AdSense provides a variety of ads to choose from and to be put on your blog. With the ad hanging on your blog and people visiting your blog, you will get paid. Getting to AdSense requires hard steps, though, you may need to search it up, and learn it.
Amazon's Affiliation Program
Amazon's affiliation program involves a person clicking on a link which will lead to an Amazon product page. If the clicker decides to buy the product, the website owner who has the link to it will earn a commision. This affiliation program could be used on your blogs and websites.
Monetization of YouTube Videos
YouTube videos could be monetized, and by doing this you could earn a lot. Try looking at some famous YouTubers, like PewDiePie, Logan Paul and others. Where do they get their money from? They earn it from monetizing their videos using ads that will come out in the beginning or pop out as an image in the middle of the video.
Clicking Ads
Some websites will provide you money if you click enough number of ads. That's it, it is simple as that.