To start selling your items to clients, you can create then easily by adding them to your catalog.
Click on Billing & Shop in the main menu. The first link, Catalog, will show you what you already have in your Catalog. The next four links (Add New Coaching, Add New Course, Add New Product, Add New Service) allow you to expand your catalog with new items. When you create a new item, it will automatically show up in your Shop where your clients will be able to buy it.
Make sure you are ready to sell by activating your Payment Settings. When you're read, turn on the Shop so that it is visible and your items are ready for sale!