Except for the edit buttons and the help, your site will look the same while you are editing it as it will later when your visitors see it. If you log in with the Simple interface, you will start with help turned on. Almost every page has a button you can use to turn help on or off. When help is turned on, there will be explanations describing what each button or field does. You can toggle help on and off depending on whether you want explanations or not.
Items represent products for sale, and sections are categories of related items.
For example, if you were creating a store selling food, you might have a section called Fruit, with three items in it, Apples, Oranges, and Pears. Item pages and section pages look different. Item pages usually contain a single big image, some text describing the item, and an Order button. Section pages usually have a series of links to the items that they contain, with smaller versions of those items' images. If you plan to have less than a dozen items in your store, you may not want to have sections at all.