As Neurotic Guy mentioned and for those some reasons he mentioned, get the certification. Even if you don't learn a single thing through it (doubtful), it will build an absolutely necessary credibility. My girlfriend was interested in becoming a trainer on the side (she was a kickboxing instructor years ago), but had no luck landing any gigs until she got NASM certified. Once you get certified, take any jobs you can to get your feet in some doors. My gf did that and now with just 1.5 years experience, she has all sorts of opportunities open to her, including leading large group workouts of over 100 people, which continues to further build her credibility, etc. And this is just a side gig of hers. Also as mani mentioned, building a strong social media presence seems to be huge these days for that industry.
I'd start with those initial steps before full on launching your own business, but never ever give up that goal of working towards your own business, especially in something that you're passionate about. It's a very liberating feeling to have control of your own life in the way that having your own business allows you to do. But you want to make sure that when you do go for it, you're set up with the background and preparation to maximize your likelihood to succeed, so I wouldn't rush it without proper certification and experience.
Curious, how old are you and what are you doing now for employment?