there are a lot of articles right here on IDEA. If you go to the articles section and look for ‘starting a business’, you will already find a lot of resources such as this https://www.ideafit.com/fitness-library/starting-from-scratch.
On this forum, there have also been questions like yours such as https://www.ideafit.com/answers/i-am-inexperienced-in-the-fitness-industr….
It certainly would be a good start to complete your profile here (always good to do as much as you can that does not involve writing a check ;-).
I would start with a really great client management software. I like www .fitify.me make sure you use one that will let you post your schedule so you can run your classes in multiple locations. Also having the ability to collect multiple forms of payment will help branding you into a legitimate business. Volunteer your services to help raise money for a cause you support, offer to teach a group class and have the charity collect the money. This will get the word out about your skills. Make sure you can organize and contact all those people that took your classes in the past. They you can do things like a message blast out to all your clients saying something like ‘you’re running a class in the park today’. I like Fitify cause they are really good to the little guy, they don’t cost much but they really bring legitimacy to any one person business like a personal trainer or palates, yoga & zumba instructor.