You can put up banners on your site, to promote your affiliate offers. Most affiliate programs will usually provide their own creatives when you sign up for their offers. All you have to do is insert the banner on a highly trafficked page (your affiliate tracking is usually embedded within the code). Banner ads in the right locations can do a great job of driving sales.
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).