Here is my candidate for most important advice: Learn about the BUSINESS SIDE of fitness with as much enthusiasm as you studied to learn anatomy and exercise science.
Before making any financial commitments, it is important to get as much advice on running a business as possible. Many states have official agencies which will help with small business to make sure that you have met all the required business paperwork. Make a business plan and have a knowledgeable person look at it. Do not let your desire over-write your better sense.
If you plan to get into business with others, make sure they have their share of the financial responsibility. Managing a studio often involves managing people. It is important to ask yourself whether you have all the people management skills necessary. It is very easy to pat somebody on the back for a job well done. It is a lot more difficult to find the right words if things did not go as you expected.