Amazon’s affiliate program is the most popular of them all. I don’t participate myself (yet) but the majority of affiliate marketers I know use Amazon because… it’s Amazon. You can review products you have used or write tutorials (eg. how to connect computer to TV) and drop an affiliate link to an HDMI cable… just a couple examples. You may want to build relationships with the manufacturers so you can get products before they’re released – giving you time to create a review before the product is launched and capture sales during peak buying times.
It can take time to build up your personal freelance business. Yet, there is more demand than ever for freelancers. So, if you want to kick start making money online through freelancing you can join one of the top freelance networks, such as Flexjobs, SolidGigs, Contena, Upwork, Fiverr, or PeoplePerHour. Sign up, build your profile, upload some samples of your work and start making extra money by doing small freelance jobs.
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.