I would start at sba.gov
This government website help me out quite a bit when I first started. Although there’s a lot of stuff they propose you look into, there’s quite a bit you don’t need to worry about. But it’s great for answering questions about legal, planning, and finances. Basically the business stuff us science-based people don’t usually know much about.
You might want to google www.thomasplummer.net.
He writes books specifically for fitness professionals who want to open gyms.
All the best!