Dressage at the lower levels isn't so much about what you do as how you do it. You might want to start with checking out the training scale or training tree For-Horses
Every horse regardless of discipline should be trained according to the training tree, but dressage riders are actually judged on it, because dressage, after all, means training.
After that, check out some dressage tests. This website Australian Dressage tests
has tests you can check out. I don't think the U.S. ones are available for free viewing without a membership to USDF. Be sure to check out the directives at the top of the test. For example, the preliminary test, which is comparable to USDF Training Level focuses on rythm and relaxation (the first two steps on the training tree) so your horse needs to not just perform the listed movements, but do it without any tension or resistance.
The novice test, which is comparable to our 1st level, expects to see that the horse has developed "thrust" (schwung on the German training tree) so the horse has to have developed some power in his hindquarters to meet the requirements of this level.
So that is what I mean by, it "isn't so much about what you do as how you do it." Between the two websites above, you should get a good start but it will take an in-person trainer to help you put that information to work.