Not sure exactly where to start testing content on your website? Below are typically the heaviest in traffic so you'll want to start your focus here. Start small and tie your strategy back to your KPIs. Use tools like Google Analytics, Coremetrics, WebSphere Commerce, and ExactTarget for email to measure your results. Most importantly - Have fun with it and get creative!
1.) Shopping Cart page
2.) Home page
3.) Product page
4.) Landing pages from search (if you are using Google Adwords) and email
5.) Taxonomy across site - this one is so important! You can use web analytics to determine if users are finding what they are looking for and easily modify your categories over time so that products are easier to find.
6.) The search box - what are people searching for? Use guided search and constantly add new keyword relationships to help people find what they need.