First you'll need to get your Federal Firearms License to become a gun dealer or/and manufacturer. There are a number of types of FFL -- google will give you a list of them and what each type of license can do.
Then, if you get certain licenses you can also apply to become a Special Occupational Taxpayer, Class 3 (dealer) or Class 2 (manufacturer).
You'll need to have your zoning worked out so you can get your business license to be running which ever kind of shop you'd have, pay your license fees (and taxes, insurance, etc.) and also you'll need to pay ITAR fees.
There are a few FFLs and SOTs here on THR. If you ever become serious about following that path, it would be worth while to talk to them a bit and see how it all goes and what it might cost.