If your online store if fairly well focused and contains different types of products that are related to each other, then deciding upon the initial categories to use for your store traffic website should be quite easy.
Because you will be using your chosen categories to create category showcases, organize your product showcases, and organize other things that you post to your store traffic site, the main thing is to come up with a list of categories that have something to do with each other.
If your store is extraordinarily focused ( perhaps you sell only victorian era hemp dresses ), then you should consider seeing if you can create sub categories for even more focus ( perhaps by style of dress, the region is was made in, etc. ).
With that in mind, look over your categories and make note of them on a piece of paper.
You will be using that list to do initial keyword research so that you can tell what names to actually use for each category on your site.
