First time I DMed, I just had fun with it. I warned my Players it was my first time and Noone cared, and in fact they were really happy to help guide me on what I needed to do, and from there I add libbed 90% of the session, and just played loose. I gotta feel for what my players wanted and did what I could to make it fun for everyone. The most prep I did for that first session was I searched for a decent inn map, a decent trail map for a random encounter, created an interesting NPC for them to get a quest from, and sent them off into the woods to find some bandits. It was very cliche, but everyone had fun. Don't ever be afraid of common tropes and encounters, or the cliche tavern gathering to get your feet wet. Those popular common scenes are popular and common because they work, after all. At the end of the day, just have fun with it, because that's what we are all here for, man.