There's a rule in business, don't start them with friends or family. Really, I'm not joking. If shit happens then the family gets broken and if it's with friends and shit goes down then friendships will be strained and friends will take sides which is not good.
You'll want a business plan first, covering every possible contingency and scenario. Maybe get someone who has knowledge of paintball games first and retain them as directors, partners, or something.
Zoning and approaching the city for a business license. Waiting for anyone to complain about the rezoning and possibly addressing the general publics questions comments concerns.
Then you're going to have to get insurance and construct all required safety management systems and have them in place (safe zones, netting, cordoning off unsafe areas, assessment of the land etc.).
You might have to hire a Geo Scientist/Geo Eng. (usually with P.Eng. association/certification/membership) or someone similarly qualified to do soil and slope stability assessments and sign off on it (safety reasons).
Then can you start constructing free standing buildings and opening up your field.
This also assumes you have liability insurance granted to you right off the bat and that you have the capital required or some sort of loan. One more word of advice, DON'T EVER EVER BORROW AGAINST YOUR HOUSE. EVER Assuming you get to this point it's a miracle but don't borrow against your house, better yet make your company a limited liability corporation (ie. you're not responsible for the loan, the company itself as a separate entity is). Keep in mind though that the courts can and will pierce the "corporate veil" if you use the entity for fraud or misrepresentation and in this case you now become personally liable and may have to forefit your house and all property.
I am NOT a business student or a lawyer or anything like that this is just wisdom that I've picked up and will impart on you.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE