former ffl holder here....
first you need to research the city, county and state laws regarding gun sales zoning.... can you operate from your home? must you be in a certain area? you may need to have a separate storefront and if so, what are the requirements for security, alarms, bars, gates, storage etc
what is your business license going to cost? what kind of insurance are you required to have? tax liability? how much start up capital will you need? what will you stock? who will your vendors be? will you pay cash or do you need a line of credit? do you have a business plan?
these are the things you need to think about before you even apply for the ffl..... because the ffl doesnt matter if you get closed the day you open or you get slapped with a tax lien or an audit..... and remember, every gun you buy or sell or fix is a personal liability for you.... so you have to make sure everything is correct. all the paperwork, all the time