To create a new Item Category, navigate to the Item icon in the upper left corner of your browser window.
Click the "+" button at the top of the left column.
When "New Category" appears in the column, double-click on the text and type the name you'd like to use for your Category.
You can rearrange the order of your Item Categories by dragging and dropping in the list. Simply select the Inventory Category, drag it to the point in the list you want it, and let go.
Once you have Inventory Categories, you can begin adding Items. Note that you must have a particular Category selected in the left column in order to create a New Item in the second column.
Also note that, by default, all of your Categories will appear in your Online Public Gallery (except for the "Fees" category that was prepopulated when you created your account). If you would like for any of your Categories to be hidden from your Online Gallery, check out this article.
Also see Additional Display Categories to see how to make your Items magically appear in more than one Category at once.