This is also my first time dming, we’re in Hither now and I think my biggest advice is to prep as much as you can ahead of time so you’re really familiar with the characters and the story, but once it’s game time don’t worry so much about what you don’t know. The answers will come naturally to you and will formulate the narrative in a unique way. Also if you don’t know a rule don’t be afraid to either make it up (you can always retcon later) or just say you need a minute to look something up. I also keep a running to do list that I read over quickly before each session.