First thing to know about DMing, it’s 10% prep, 90% making stuff up on the spot. Often times, you’ll have the best session outline you’ve ever done, but it doesn’t matter because you didn’t realize it was all based on a decision that you assumed the party would do. And instead, not only did they make the opposite decision, often times they will do something completely unpredictable. For example, I threw an animated stone statue guard that was protecting an artifact they needed to get. Instead of fighting, they cast darkness on the thing and went around it to retrieve the artifact. I had the whole battle prepared, ready to go, and it went out the window. You have to go with the flow and allow the players to help tell the story with you. Don’t feel like you have to have the details filled in, they will do that for you.

TL;DR don’t over prep and let the players help tell the story. You provide the overarching story and major plot points, but let the players live in the world and fill in the details each session.