I just responded to a question on facebook someone had about wanted to open their own gym and where to start. Since I'm kind of in the thick of that right now I realized I've got a lot of first hand knowledge on the subject and thought it might be a good idea to post some of it on here.
So, wanting to start a gym. The first thing you really need is a student base. I've been teaching out of a local gymnastics school for about 3 years now and I've got just over 100 students. Everything comes down to money when you decide to make a business.
The space to rent out for a gym usually will cost about $1/square foot a month, so if you want a 5000 sq. ft. facility (pretty typical 50x100 feet or so) it's going to cost around $5000 a month. You could get a smaller gym, but smaller space is going to be more limiting. The Tempest Freerunning Academy is around 7000 sq. ft., the space we're looking at right now is 7840 sq. ft. You may also have to pay utilities in the space which they usually estimate at about $0.10/sq. ft. a month (so about $500 a month in a 5000 sq. ft. facility. The space will also have to be fully up to code and registered as an assembly occupancy for the amount of people you can have in it. With that comes the codes on safety like sprinkler systems and air ventilation as well as accessibility for handicapped people. Since you'll probably end up renting out a warehouse space with relatively high ceilings, this means the space will probably be registered for storage and not assembly (this is the case with our space). The sprinkler system will probably be fine, but the parking may be an issue as well as handicapped access depending on the space. A bigger issue is the air ventilation since storage space is not required to be well ventilated and this means you may have to put in an HVAC unit (which is what we're going to have to do and it's going to cost about $30,000)
With that you'll need equipment, you can build a lot of stuff out of 2x4s and plywood. We're planning on spending about $5000 right away in building materials for everything. Metal scaffolding is actually really cheap right now since the housing market died, all of the bars for our entire gym is going to cost about $850. But if you want any sort of mats, those are expensive. Just look at something like
www.tumbltrak.com to see what I'm talking about. We're going to end up spending about $15,000 in mats. Also flooring, as awesome as it would be to keep the floors just concrete, there's no way any insurance company is going to allow that. So we'll be spending about another $10,000 on floor matting. You also need to consider the administrative side of it all as well. You'll need a computer and software (Quickbooks for the business is about $200 and a Class Manager system for registering students and taking payments is about $600) as well as probably a printer/scanner/copier, filing cabinets, desk, etc.) Also the random stuff you may want like a sound system to play music in the gym (or a projector tv like we're planning on getting to play motivational freerunning videos on a 12' screen on the wall all the time

Speaking of insurance, you'll need to get that figured out too. I recommend talking to a private insurance broker (if you go to a gymnastics school and ask, they may be able to hook you up with someone) since they can talk to lots of big companies and know how to deal with them. Trust me you do not want to deal with a big insurance company directly. You can pretty much expect to pay about $1000 a month for insurance. And that's not just liability insurance but also workmans comp, unemployment insurance, and all that jazz as well.
When you add everything up, if we charge around $75 a month to our students, we need about 120 students to keep the place running and to not actually make any money to live off of. Plus we'll have to get a small business loan to buy all the stuff to get started, and paying that off month to month adds even more to our operating expenses. And our instructors will need to get paid (plus as a business you need to pay a bunch in payroll taxes as well).
So I guess you kind of have 2 first steps to take. First of all, what kind of teaching experience and student base do you have? Without students you have nothing. Second, what kind of business experience do you have? Can you write a business plan, apply for a loan, talk to an insurance agent, talk to the state and inspectors about business permits and registering everything? Remember once you change a hobby into a business, everything changes. It's all of a sudden not as much about the love of the sport and all those noble goals, suddenly it's more about paying the rent and putting food on the table. Of course it is still about teaching more people as effectively and the best way you can, but if you don't make enough money to keep your facility open, there's no point to all that.
I don't want to sound discouraging, it's definitely possible, but it is a lot of really hard work. I've been planning this for the last 4 years and it's just now happening. I've learned a lot along the way though and I'd be glad to help anyone out as much as I can. So if you have any other questions feel free to ask.