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.
Banners – after testing them out I decided to take down my banner ads since they looked salesy and weren’t working like my affiliate links did. They’re easy to throw up, but distracting and probably won’t get great results. If you try them, be sure to show specific sidebar banners based on the type of content people are reading on your blog (for posts that fall under my SEO category I would show a banner related to SEO, and for posts under my website speed category I would show a different banner). You can do this using a plugin like Widget Logic.
Affiliate marketing is often touted as a "get rich quick scheme" by shady sites offering pyramid schemes that promise quick cash for little effort. Make no mistake—successful affiliate marketers like Pat Flynn of Smart Passive Income put in a lot of effort toward building an audience and creating quality content that will bring in sustainable passive income. Expect to do a lot of legwork up front—but if you play your cards right, you can build a solid source of revenue over time.