Sorry to keep adding posts; my brain keeps thinking of things after the fact.
Assuming your area is like mine, people will be friendly and happy to see someone trying dressage. If your day goes badly, you might expect a funny comment on your dressage test (I had one about some airs above the ground that were not meant to be in the test - the maestro would be jealous), or at the tack check.
Don’t worry if your tack and clothes aren’t exactly what everyone else is wearing (provided it’s legal). They’ll be happy to have you there and don’t hesitate to ask questions of your fellow competitors or the steward. While the warm-up ring can sometimes feel a bit intense with some riders ultra-focused on riding movements no matter whose line they might be cutting through, over all the dressage community is much less intense and much less cliquey than what you’ll be used to.
Don’t be surprised if a competitor wishes you luck while you pass on the way in/out of the ring. They’re a supportive group!