Lots and lots of lessons, I started riding when I was 6 and always took lessons. Also rode a lot of different horses.
I think probably the fastest and most effective way to learn if you eventually want to train on your own is to get a job as a working student somewhere. You learn all of the aspects of horse care/barn management and get unlimited lessons and lots of free education.
Watching a lot of videos and reading a lot of different books helps too, but is hard to correct a problem you don't know you have (sitting off to one side, for example).