Many people (if not most) who work as fitness professionals begin as fitness enthusiasts. The first step, as I see it, is a change in perspective: what you learn will no longer just be about what works for your own body, but what will work for the clients with whom you work, and what you do when you work with a client or student will not be about your own fitness, but about them.
There are many training programs. Some people do a university degree. There are a number of programs available, with different foci: wellness, human kinesiology, exercise science, even nutrition. A degree is not necessary to become a trainer, but it is an excellent way to learn what you need to know, and to position yourself to command respect as someone entering the profession. You could decide to do a series of workshops, or even online training. Whatever you choose try to have some hands on training if possible, and try to choose the best program you can afford, rather than the cheapest and fastest.
Once you finish whatever training you decide to take you will need to take a certification exam. There are quite a number of these as well. Some are well respected in the industry, and some are not worth the paper on which they are printed. Here is one source to start to compare options:
You might also see if you can shadow a working trainer, or find an internship, or perhaps even take some work at a fitness facility in another capacity, so you can start to learn from the inside. Some companies even offer training.
One who works in fitness needs to have a strong understanding of anatomy, physiology, kinesiology, psychology as it relates to things like motivation and adherence, as well as things like legal issues of running a business, fitness technology, injury prevention, special populations, the effects of medications, and functional training, as well as a thorough understanding of the principles of designing and individualizing an exercise program.
And of course, one has to continue to keep up with industry trends and current research.
Fitness is a great place to be. I wish you luck.