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.
A more complex system, pay per lead affiliate programs compensates the affiliate based on the conversion of leads. The affiliate must persuade the consumer to visit the merchant’s website and complete the desired action — whether it’s filling out a contact form, signing up for a trial of a product, subscribing to a newsletter, or downloading software or files.