Once you are a competent rider, you should get a pretty solid horse.
From there, your trainer should help you work with your horse. The lessons should be less about how you're sitting, and more about how you're influencing your horse.
Obviously if you just got your seat down, you're not going to be ready to train a horse from the ground up. You'll probably want a horse that has been trained in whatever discipline you ride in. With that horse, you will learn how to make him move correctly and keep him working right..
Sorry if that makes no sense, I had a hard time explaining it.
Every great dream begins with a dreamer. Always remember, you have within you the strength, the patience, and the passion to reach for the stars to change the world. ~Harriet Tubman