Near the top left corner is a drop-down menu containing all customer names which already exist in your database. To start you should select the customer from the list then, near the bottom of the window click the button labeled "Add Inventory Items" to bring up a list of all items in your inventory as shown on the following page.

Note: It goes without saying that you won't be able to process orders unless you've already added customers to your database. (See the section in this user guide for maintiningg addresses)