If you are wanting to learn how to train a horse, then you need to get a trainer that is willing to work with both you and the horse. That gives the horse the best chance of getting proper training...and it minimizes the chance of you getting hurt by making a big mistake just because you didn't know it was a mistake.
Believe me, sitting on a horse and having him just stand there doesn't always mean that he'll be a cinch to train. I've been on lots of horses that would stand there calm as you please or walk around like an old plug with a rider, but the instant you asked for something more, you suddenly had a rodeo bronc on your hands.
Someone without training experience wouldn't be able to recognize the signs of something about to happen...and wouldn't have any idea of what to do when it did happen.