One of the most educational and fun ways to earn money online comes from buying a product or a service, using the product or service and then writing a review about it where you use an affiliate link to the product or service. This way you provide unique content to your blog from what you have learned from using the product or service. By providing unique content to your blog, you have a greater chance of better search engine ranking and for other people to link to your blog.
I think that one of the most interesting ways to do this, and probably one of the easiest ways as well, is to become an affiliate of Amazon and then buy some books about topics you really enjoy. Then read the books and do a review on your blog with a direct link to the book at Amazon including your affiliate id. This way you will read books about your favorite topics and earn money from it.
Amazon has been around for a while and they have become a really big company that you can trust and that your visitors also do trust. It seems like most people who are active online are customers of Amazon already, and if they find your review interesting it might not be hard to actually sell them a copy of the same book.
To get you started earning money online, you could join Amazon as an associate today and write one review a week or so.