the short answer
To become a freelance personal trainer, earn a recognized certification, register as self-employed, and put professional liability insurance in place before your first paying client. Then pick a niche, set your rates and package a clear offer, find your first clients through your network and referrals, and choose software to deliver programs, track progress, and take payment.
This article is general information for trainers, not legal, financial, or medical advice - self-employment, insurance, and certification rules vary by country and change over time, so verify the specifics for your situation.