Don't actually quit your desk job... yet
Once you've picked a quality training certification, keep your desk job while you study for your exam. Give yourself time to go through the study materials (it shouldn't take more than a few months). If you don't have a formal background in anatomy, physiology, or exercise science, you might be surprised how much actual science is involved. Believe it or not, training requires more than bellowing, "Stop whining!" and, "Five more reps!"
If you're convinced you can't stick to paper-pushing while studying for your exam, look into getting a position at a gym or studio in a support capacity. It'll give you an "in" when you're ready to start training, and you'll learn more about working in a gym environment. Of course, that would mean working for what usually amounts to part-time minimum wage, so you might want to wait until you're actually certified to make the switch.
It takes time to build a clientele and reputation as a trainer or instructor, and many trainers start the journey part-time. Try picking up a class or two at a local studio or begin working evenings at a gym. You can start saving your extra cash to support your career change when you decide it’s time to pull up your stretchy pants to help other people get in shape.
Like pretty much every other modern job, use the internet to your advantage
These days, trainers are also entertainers. While you shouldn’t adopt an over-the-top personality just for the sake of your training career, it’s important to think about how you develop your brand as a trainer. What do you stand for? What types of clients do you hope to train? How do you plan to help people meet their goals?
For instance, if you want to appeal to a rich clientele (lookin’ for the money, honey!) you need to market yourself as a luxe commodity available whenever your client needs you, however they need you. Do they need someone to rip food from their shelves and show up for 4:30am in-home sweat sessions? Boom, you’re there, always looking every bit as hot and rich as they are.
On the other hand, if you want to work with stay-at-home moms (who doesn’t want to train clients only between 9am and 11am Monday through Friday, amiright?), you may want to focus more on fast workouts, getting body back after baby, and realistic health-focused workout plans.
And if all else fails, you can always cross your fingers, post shirtless pictures to social media overlaid with motivational quotes, and hope you become Instagram famous. I and all other legitimate trainers will hate you from the depths of our souls, but who cares? If you look hot, it’s worth a shot.