Hi. I just joined today. In the auctiva store, how do I set up the store categories so that it matches my eBay store?

You can't have store categories as such, but you can do custom searches.

IE category name - Auctiva
Search String - Auctiva

so when a buyer clicks on the Auctiva "category" it does a search through all your items to find any listings you have with Auctiva in the title.