There is not one certain way to train a horse, but there are definitive ways to care for a horse so that you and your horse are safe. Horse training is really not something you can go to school for, although there are so many horseman that offer their services so a young person has the opportunity to learn from experts in their breed or discipline.
When I started I took riding lessons, then got a job grooming on farms in Kentucky, then got a job grooming in California on the racetrack, then got a job as a foreman and eventually an assistant trainer. I was an assistant trainer for 6 years before I took out my license to be a thoroughbred horse trainer. So what I am saying is; don’t rush it and postion yourself around the best horseman you can find.
There are now colleges and universities that offer Equine Managemant programs, but I believe to become a trainer in a specific discipline you will need to work with accomplished trianers within that discilpine or breed. Start at the bottom and work your way up and it will make you a better horseman.