the short answer
To start a personal training business, earn a recognized certification, register the business and put professional liability insurance in place, package your coaching into a clear priced offer, win your first clients through your network and referrals, and set up a software stack that delivers programs, tracks progress, and takes payment. Going online-first lets you skip a physical studio and coach clients anywhere - which is the lowest-overhead way to start and the easiest to scale.
This article is general information for coaches, not legal, financial, or medical advice - registration, insurance, and certification rules vary by country and change over time, so verify the specifics for your situation.