Let's look at the affiliate program of a fictional company called Daisy's Emporium. Daisy's Emporium sells all kinds of things online for a very reasonable price. Everybody knows about this store, and almost everybody has made a purchase online through this store. It's a trusted store. On its website, Daisy's Emporium mentions its affiliate marketing program and how it pays each affiliate 10 percent of each sale they make. That's a pretty good percentage, especially since most customers of Daisy's Emporium make a purchase of at least $100. A 10 percent commission from a $100 order is $10. If you spend one hour working on your affiliate marketing and make five sales, then you could potentially earn $50.
Music may perform better than books and other products, mainly because you can listen to the clips of an entire album in roughly 10 minutes and get a good enough feel for it without buying it to write a short review. If you have another topic that you're passionate about, great, but make sure you have a unique angle on the topic. People can get reviews about a lot of those consumer products anywhere. You need to give them a reason to visit your site.
If you are building a site that has the potential for information that will never age and remain useful for your audience, you have the opportunity to create what is known as evergreen content. It's important to carry out extensive keyword research before planning any evergreen content for a site like this, as your site could hugely benefit from the proper usage of keywords within such content. 
The topic you choose must have enough depth that you can create a lot of content for it. This is important for building an authoritative site, for search engine optimization, and most importantly, for the end user. If you don't have enough content about a topic, you're not going to be taken very seriously as an authority on the topic and it's unlikely you can convince someone to make a purchase from you. 

After being accepted into an affiliate program, marketers receive a unique URL that includes their affiliate ID. They share that unique URL with their subscribers, site visitors, and social networks via text links or ads. When someone clicks on that link, affiliate software records that click and any resulting product sales in the affiliate’s account. When commissions reach a pre-determined threshold, the affiliate is paid.
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