Starting a gym it’s no different than starting any other business. Having a solid business plan is a must! One more thing to consider (I know you want only one thing to name here, but it’s impossible when starting a business) is to do some research of your competition and determine if there is really a need for a new gym. Gyms today pop-up like Starbucks coffee shops and this is not good news for those who are looking to open one (market saturation). I don’t own a gym, but these two factors are the ones I would consider first.