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.
We want what we want, when we want it, isn’t that right? Isn’t that why, as a culture, we’re so impatient? Isn’t that why we have fast-food joints around every corner? Isn’t that why we demand instant service before taking our business elsewhere? Isn’t that why we complain when we don’t get our way as quickly as feasibly possible? Yes, it most certainly is. We’re simply a byproduct of our own culture and society.
Next, you’ll want to pick a WordPress theme from somewhere like ThemeForest, Elegant Themes or OptimizePress. This is the barebones design of your site, which you can then customize with your own branding, copy, and images. That being said, you don’t want to cheap out. It costs less than $100 to buy a theme that will make your website look professional (and you can upgrade to a completely custom design once you get the business going).