ok well dont ask me about levels, im not sure when you get what etc.
To make a "house" in to a "shop" first you need to make the house public, and add a vendor.
When you make the vendor you will be given the options your level allows you to create, vendors, droids, npc's etc.
That will create a vendor in your inv. You then drop the vendor where you wanna place it, intiallise it and your good to go.
Best way to get teh hang of it is to make one and mess around with it till you get the idea of what you can and cant do.