First you'll need to look at the zoning of the property. You'll want to make sure the the property is zoned for commercial/industrial use with a usage permit for recreational activity. If you don't have the proper zoning, you'll never get the proper insurance for it.
Second you'll need to go to the county the property is in and get it licensed. Currently in Ontario there are specific paintball field licenses that are granted and come with specific conditions. The same applies to airsoft.
When you go to the county, you'll need a solid business plan with a proper site plan. The site plan will need to include 'buffer zones', safety areas, a copy of your waiver, parking, staging areas, etc.
This is just to start. Be prepared to spend between $20,000.00 and $100,000.00 for just the set up and licensing before you even have a legal game played.
I've worked for years with paintball fields to get them set up properly, and this is what it takes. Hope the information helps, if you need anymore, feel free to pm me.
The Tier One Tactical Operator
Tippmann Arms M4A1
KWA USP .45, KJW P226, KJW hi-cappa