If you’re doing this part time, one client at a time, not sure it’ll be worth it and you’ll have to hustle to find, keep and replace clients. My suggestion is to think of doing group classes. Particularly outdoor training. You’re going to need personal indemnity insurance though in case anyone gets hurt, you don’t want to be sued and lose everything. Ideally you have a PT certificate or training of some kind, that’s time and money.
Another option, which I always recommend is to train as a masseur. They charge more, easier to get clients and it’s also a portable career. Of course you have to pay for that training too. And have insurance.
Oh, you’ll also need to set yourself up as a business so paperwork and taxes.