Dressage is a French term, most commonly translated to mean "training". Its fundamental purpose is to develop, through standardized progressive training methods, a horse's natural athletic ability and willingness to perform. At the peak of a dressage horse's gymnastic development, it can smoothly respond to a skilled rider's minimal aids by performing the requested movement while remaining relaxed and appearing effortless. Dressage is occasionally referred to as "Horse Ballet". 

In modern dressage competition, successful training at the various levels is demonstrated through the performance of "tests" of prescribed series of movements within a standard arena. Judges evaluate each movement on the basis of an objective standard appropriate to the level of the test and assign each movement a score from zero to ten - zero being "not executed" and 10 being "excellent". A score of 9 is considered "very good" and is considered a particularly high mark, while a competitor achieving all 6s (or 60% overall) should be considering moving on to the next level.

             You should start out by getting a trainer who can teach you the dressage basics and later more complicated moves. While that is going on you should get some books on dressage and movements and read them. Also you can work by yourself and really perfect things such as transitions, lengthening and collecting, bending around circles and corners, getting your horse elastic on both sides, and then train yourself on some basic movements. I hope this helps!