If you have a item .php file, put that file in Sources/shop/items (as xfollowthereaperx mentioned). It will then appear in the Add Items dropdown list.
If you want to make your own item, you'll need to know PHP. However, there's some basic items included with SMFShop that you may be interested in (Display Message, Email an Admin and File Download are the main ones). Using these included items requires no PHP knowledge (basic HTML knowledge is needed, though)