SCORE is a great service.
The service that Deserve mentioned that you might find at a local university is the SBDC network (Small Business Development Center).
I happen to be a Director of an SBDC. They offer one-to-one counseling free of charge. They can really help you with your business plan...and also pro-forma financials. If you tell me where you are located, I can tell you where the nearest SBDC office would be.
What you have so far is certainly
A piece of the puzzle. But the big puzzle, the business plan, has far more pieces. And even the smaller puzzle, the financial projections, will require a lot more information.
Before you can honestly go much farther, you will need to understand more specifics regarding your initial costs and your monthly costs. Specifically, you need to understand more about how much your site is going to cost!
You might call a commercial real estate agent in your town and try to find out a good range of prices you'll pay to lease commercial real estate. You probably have some decent idea of how many square feet you would need to open the kind of facility you will want. If you think you need, say, 3000 square feet, you might find an agent that tells you that type of space typically goes for $22 per sq.ft. So you can figure your rent at $5500 per month. ($22 times 3000 sq.ft. divided by 12 months...and, oh, by the way, there will probably be more "triple net" charges, but at least you can START somewhere.)
As for your INITIAL layout, depending on your local market, of course, most commercial real estate owners are basically not going to lift a finger for you. If you need to install locker room facilities, for example, you're probably going to have to come up with the funding to do that. What is that going to cost? Maybe $30K? Maybe more? It depends on the site, obviously. Some sites may require more plumbing, more electrical...some structural work....you just never know until you know.
That brings up an interesting point... Let's just say that you're going to need...oh, how about a nice, round $100,000 up front. You're already close to $50K and you haven't factored in any leasehold improvements yet. You're also going to need some working capital. You have to be able to pay the bills your first few months in business while you build a clientele. I don't know this industry that well, but I'd suspect that $100,000 might be a reasonable figure to get a gym started....or actually on the low side of reasonable, depending on where you are located and what type of clientele you are looking to serve.
Where are you going to get $100,000???
Seriously, there is this funny mythology in this country...we seem to think that if we're smart and we have a good idea, the money will be there! I'm here to tell you, the number one reason that people don't get small businesses off the ground is that money does NOT grow on trees. Do you think you can write a business plan and walk into a bank and borrow $100,000? Uh...almost certainly not. Unless you've got mad connections, baybee!
A bank is going to want three things from you:
1) They are NOT going to be the only investor in your business. That means you had better come up with at least 20% down yourself...or maybe with a group of investors (get your elevator pitch worked up!).
2) They want collateral...full security for the $80K they are going to be lending you. Guess what? Leasehold improvements are NOT something the bank can sell! All that work you want to do on the locker room? Sorry...the bank can repo it and sell it, so they are not likely to loan on it. In fact, the market for used equipment is probably pretty soft. You'd be lucky to have a bank collateralize $20K on a bunch of used equipment and teevees.
3) They want you to prove that you can pay for the loan out of the operating cash flows of the business. So you WILL, as John mentioned, have to spend a lot of time researching your market. Who is your competition? How do you differentiate yourself from them? What products/services will you offer? How much will you charge? At what point do you break even? How much market share will you need to capture? Specifically, how will you design your marketing communications to target the demographic that you need and how can you be reasonably sure you will be successful?
So how much money do you have right now? Can you borrow against a 401(k)? Can you tap your home for equity? What are you going to risk, here? My rough take is that you'll be lucky to get a bank to pony up a third of what you'll really need. (I'm basing that number on my rough view of your collateral position.) You got seventy large? If not, it's time to find INVESTORS.
For many industries, this is the make-or-break skill for an entrepreneur. How good are you at asking your rich Aunt Sally for money? You say you don't have a rich Aunt Sally? Well, then it takes even MORE skill because you're going to have to hit up a bunch of strangers. They say the money for start ups comes from The Four F's....founders, family, friends, and FOOLS. When you get down to fools, you're going to have to be pretty persuasive.
Of course, there ARE other options. Fitness centers are a tough business to be in. I'd suspect that many of them are started by people with more money than sense...or that people get into this business because they like to train and they don't realize that OWNING a gym is nothing like training a client. Depending on where you live, you can possibly find somebody who is selling one right now at
www.bizbuysell.com or some other similar business brokerage website. Sometimes you can get a good deal and you can find something like this for less than the market value of the assets just because somebody wants OUT. Of course, then you might be fixing something that's broken....better get your Marketing hat back on.
You can also lease your equipment rather than purchase. That reduces your collateral position and may make you less bankable. But it could also dramatically reduce your start-up costs. Since you're likely to be talking to investors, at least this might allow you to ask for less money from them.
Now, don't get me wrong...I know this might come off as somewhat discouraging. I certainly don't mean to be that way. I work with entrepreneurs like you EVERY DAY and I find that it is EXTREMELY rewarding when they succeed. The thing is, you DO need to understand some of the challenges that you will face.
People like JP have stared down these challenges and lived to tell the tale. I can't tell you how much respect I have for people who turn their passion into their business. It is NOT EASY to do! (Especially in this field.) Of course, again, people like JP might be able to tell you some more dollar-figure specifics than I can tell you. Entrepreneurs generally know their industry a lot better than I know their industry.
They say that every small business, in order to succeed, needs to master The Three M's:
Management--can you manage your enterprise or put together a team that can do it?
Market--is there truly a market for your product/service? Can you communicate with that market?
Money--Do you have it? Can you get it?
I really didn't even address that first M in this post, but hopefully I've given you a little bit of a flavor for what you might face with regard to your Market and Money.
(Wow...every time I post in this forum, I get long-winded! I'll stop now.)
Opening My Fitness Center
No wonder everyone thought it was such a short list! 
Here it is......
