First off, get age verified. Where are you located? You need to find people in your area.
Hardcore in Penticton, BC does something very similar. Maybe try contacting them for ideas on the field. However their rental guns are walmart grade.
If you want to host milsims, I'd suggest you attend a few. Operation Mason Relic in Claybank Saskatchewan is a good resource. There is a large diversity of players there. Usually between 200 - 300.
Have a skilled gun doctor on hand. Or find one close by that'll help you out. The ones that are worth your time charge for their services. So find someone reputable.
Study all of the legal implications. You will have to set up safety rules, waivers, ...ect. Find out the style of play of most of the local teams. That will help your set up. You'll need things like chrono's, BB's, ect.
Put a lot of effort into your field. Especially if people are paying to play. Visit some fields, or look online.
As far as running a shop goes. You'll need to find a supplier that carries what your looking for. Someone like "Spartan airsoft" for example. Figure out what guns you want to sell/rent, and pick your best suited supplier. You'll need all the proper business licenses, importers licenses and the like. Really look into the legal side, as there is a lot of it pertaining to airsoft. I'd check out a few shops first hand to get an idea.
There's a lot more to it, but I hope this helps. To run something like this it would be a good idea to know airsoft up and down. Learn about guns, brands, gear, laws ..ect. Airsoft Canada is s great place to learn about it, but experience a bunch first hand. Maybe a well known local team will let you pop out a few times.