You're going to be doing product reviews and recommendations, so pick a topic that you enjoy and about which you can demonstrate some expertise. Choose a narrow enough niche to be distinctive—for example, bands from your city, left-handed guitarists, music for a certain kind of dancing, authors of a certain religion, books about business, or arts and crafts resources. If you can't stay passionate about the topic, that will show.
Affiliate marketing pulls together marketers who want to advertise and publishers, sometimes called influencers, who want to promote products and services and get paid to do so. Publishers place customized links to things they want to promote within their website or social media content. Then, they get a portion of the sales as a thank you for their promotion.