I’ve been Dming since 1st Edition and my #1 rule is to not over prepare. This requires an ability to improve a bit, and be comfortable with the fact that mistakes will happen, and be sure your players understand that. Also, for your first outing, don’t do anything homebrew, it’s fun and one of my favorite parts of the game, but it can also easily break the game if you don’t know exactly what you are doing. The main thing is to have fun.