Work with the end in mind
I've been a trainer for about 13 years, so here's my 2 cents.
Start with then end in mind. What are you hoping to accomplish? How much do you want to make? Who do you want to train? What hours and how many hours do you want to work? Group classes/one on one? Do you want to work for someone else, or for yourself?
Get the certification out of the way. You'll need this to get started, but the real learning comes from your passion and continual learning from sites like Per. Menu, and your time with clients, workshops, and experience.
I know for myself that the business and marketing has been the biggest challenge. It's one thing to get great results for your clients, but if they aren't spreading the word and you aren't filling your pipeline with new clients, you're going to have a hard time paying your bills. I've found that my clients who have successful businesses are very willing to help with business and marketing ideas.
One book that you may want to check out is "The E-Myth" by Michael Gerber.
Just as your performance and physique results came through calculated practice and implementation, so will your business.
When you get it right, this can be a great profession.
Go for it,