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.
MoreNiche is one of the few affiliate networks that offer free affiliate training and ongoing personal support in this way. Many people who don’t know about our network currently pay out hundreds, if not thousands of pounds on affiliate training courses, seminars and even personal mentors each year. But they only learn the same information we give away for free once you’re working with us!