Welcome! I’m a recent transfer from HunterLandia myself. In general, I’d agree with the saying that dressage is “riding for nerds.” There’s lots of theory underlying the discipline (lots of good books out there!), and at least in my experience I was very impressed with the skill and knowledge level of the average amateur rider at even local unrated shows. The camaraderie is also better than hunters, I think because the explicit scoring allows you to compete against yourself as much as you do against others. I loooove getting judge comments back, and after scribing a few times I can say that most judges are honestly really rooting for you, which just leads to a better atmosphere overall.
One good resource is to search for youtube videos with judge’s comments. That’ll help you develop an eye for what is rewarded and not. In addition to reading the rule book, read through a couple of tests at different levels. Pay attention to the directives (what’s expected of a lower level horse compared to upper level? which words keep showing up?) AND the score weighting (what has coefficients and counts double?). If there are clinics or schooling shows in your area, see if you can maybe volunteer and/or scribe a bit, too. Like, I could have told you that showing a clear difference between movements was important, but writing “show more” a bajillion times made an indelible stamp in my mind. There are guides for scribes on USDF, and even if you don’t want to scribe anytime soon it gives you an idea of what kind of comments crop up.
In terms of riding practice, measure out at least a few letters in your arena and some 20m circles (especially if your usual arena isn’t standard court sized) and practice paying attention to geometry. Riding practice tests is really fun too- even if you’re on a baby there are plenty of w/t Intro tests out there! And feel free to pick out bits and pieces of higher level tests if you think they’ll help you learn or train something. While I’m not a particularly competitive person, test riding helped me understand just how precise my riding could be- like exactly how many strides out do I need to start thinking about going from free to medium walk?