Take advantage of marketing tools whenever possible. Many affiliate programs offer useful resources to their affiliates aimed at helping affiliates make more sales. These range from monthly newsletters highlighting upcoming sales, tips or affiliate case studies, Facebook Groups just for affiliates, private webinars explaining marketing strategies in depth and more. Ultimate Bundles does this exceptionally well.
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).