I have been personal training for 20 years. I have been a good personal trainer for about 10. It takes that long to get good. The number one thing is you have to be safe. You have to know as much about injuries as a physical therapist. As stated above, you have to give the client what they want, this does not always mean hard workouts or even getting in shape. You have to be able to work with young 'bodybuilders' just as well as 85 year old women. Again, you have to be safe, this is by far number one. You will never make money in corporate gyms. You have to not bullshit your clients. You have to be able to train doctors with enough knowledge to know what you don't know. You have to know how to work around injuries that can't be fixed. There is no certification that will even teach you 1% of what you need to know. Only a lifelong passion for fitness and constant learning about your own body will bring you even close to the knowledge you need to be a good trainer, then on top of that you need thousands and thousands of hours of experience working with people. You have to be a naturally good communicator, and you have to hone your communication skills to an extreme level. Good trainers are one of the rarest things in any profession, and if you are truly GOOD, you will have no problem gaining the clientele you need. You also have to be a natural leader. You will hit a hard cap to the amount of money you can make because you only have so many hours to invest, and can only charge so much.
Probably easier to become a hedge fund manager and make millions of dollars...