Find a decent enough venue,start asking around different vendors if they would like to set up a stall,explain this is your first one,they have all had plenty of experience with this sort of thing,ask other vapours that have been to one what were their experiences,what was missing if anything,advertise.
That's a good start for you,think about how many may turn up,will you need a refreshments stall? Toilets obviously,what time of year is it? How are people going to get there? Trains? Airports?
Cover all angles,kids? Animals?
Its a lot of planning and organisation,but you seen capable of that.
Good luck x