Owning a business will require a lot more time than you have available at the moment. I agree with the others that the best option for you right now is to get a job at a local gym and expose yourself to as many clients as possible. It will be a great experience which you can use later when you’ll have you own business. Starting and operating a business is very complicated process and if you don’t have the experience or the right people to help you set up your business, it will be difficult for you to grow it. I’m not saying it can’t be done, but if you are going to do it it’s better to get it done right from the beginning.