Welcome!
I would think you've got two options: buy out an existing store, which would require finding a willing seller and money but not much else, or start from square 1 - get business license, get business firearms license, location, inventory, staff.... other than the firearms business license I doubt it's much different from any other retail store unless you're in a major city (a lot of them have rules that make it hard or impossible to get a business license for a gun store).