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.
The products and services you will be promoting to your audience must be relevant and good quality. Make sure you believe in them and know everything about them, because this will be crucial to you delivering the sales pitch to your audience. You need to build trust with your audience so make sure the products and services you choose to promote are trustworthy enough.