Theme – you don’t need a special theme for affiliate marketing, you probably just need a blog. I recommend StudioPress themes since that’s what Yoast, Matt Cutts (from Google), and I use. Matt Mullenweg, founder of WordPress also recommends them. One of the biggest mistakes I made was using a theme from Themeforest… since they’re built by independent developers who may stop making updates to their theme. This happened to me and I hear horror stories all the time about people having to switch themes and redesign their entire site. I’ve been using the same StudioPress theme (Outreach Pro) for 3 years. Their themes are lightweight (load fast), SEO-friendly via optimized code, secure, and they have a huge selection of plugins for the Genesis Framework and an awesome community in the Genesis WordPress Facebook Group. They include documentation for setting it up and will serve you for many, many years.
Always disclose your affiliate relationship. Most visitors will probably understand that graphic ads will lead to your getting paid, but if you write a review or use an in-text link as a recommendation, you want your readers to know that may lead to compensation as well. This ensures you retain transparency and trust with your readers, but also, it's required by the FTC's endorsement rules.