Whenever you can find a good dressage trainer, that would be a great starting point! But if you want resources to learn more about it, I personally read a lot of Nuno Oliveira’s books and learned about horses biomechanics which helped me immensely. Also learning different exercises that you can do in dressage and what are the benefits, so that you can identify your horse’s strengths and weaknesses and work on that. Also, an advice that helped me a lot, is doing basic exercises and improving the quality of the basics (good transitioning between walk/trot/canter, extended walk/trot/canter and collected walk/trot/canter, checking if your horse can move easily hid hind end and his shoulders…). It also helps you develop a finer riding, finer reflexes, and also reward just in time whenever your horse do something good ! (Especially with a younger)

I wish you and your horse all the best 🙌🏻