Just a little side note here - Gus bare in mind that you will not be riding 'pretty movements' and elegant, collected horses just yet. It is a common misconception with people that are not familiar with dressage to think that they'll go straight into learning movements, this is not what dressage is. Dressage is the training of a horse from the very basics, and the movements are ridden later in the horse and rider's training to test that they have established correct basics, and to encourage further strength and training in the horse.
When you start dressage lessons, you're not technically riding dressage as such while you are learning to adjust to english. The instructor will be teaching you a correct english/dressage position, different aids for basic walk, trot, canter, halt and turn. It will be like going back to being a total beginner rider to start with. Once you have a balanced seat in a dressage saddle, can follow the horse's movement and effectively influence it , then you will start getting more into how to improve the horse's way of going. This is when you start true dressage - the training of the horse.
Therefore, all of that garbage you heard about it being difficult and all about invisible aids and sitting pretty, discard it! Yes, the goal is for the rider's aids to appear invisible however when you are starting out, this isn't going to happen. It takes years of hard work to get to the point where you can influence a horse in total harmony with just the tightening of a muscle or small shift of weight. Each aid for a movement, transition, turn etc. is different but so slightly that you must be exactly spot on with it's application. Again, this all takes a lot of hard work and a long time to master!
I just had to add all of that, because I know people who have said they want to do dressage, they go and get some lessons and are sorely dissapointed when they're not riding half passes and pirouettes within the first 2 lessons!