When promoting affiliate offers, just make sure you are fully aware of all the terms and conditions attached to your affiliate program. Some programs can be strict about how they allow you to promote their products. For example, some may limit you to banner ads and links only, while others will allow you to use paid advertising, but won't allow email marketing.
StudioPress itself is somewhat of a niche product as it is targeted to existing WordPress users who found setting up and managing a WordPress site too difficult or time-consuming. StudioPress prides itself on being easy to use, but their main claim to fame is that their hosted websites are “faster and more secure” than other WordPress hosting companies as well as using the “Genesis framework” which is supposedly more SEO friendly than other WordPress builds.
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.