On-site Analytics (preferably Google Analytics) – It’s free and can be easily integrated with WordPress and other website platforms. Google Analytics lets you see bounce rates, sources of traffic, keywords used to land your site, and a whole lot more. You can use this information in developing your topics that your readers like best and improve traffic of your site.
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.