One of the biggest pitfalls for novice game masters of any tabletop RPG is knowing how and when to say no to players.
Learning to improvise in Dungeons and Dragons: How to salvage a session when your players are Chaotic Neutral
Tabletop players are unpredictable -- that's what makes Dungeon Mastering fun, ultimately.
Here's a little secret -- I don't do a lot of formal planning for running my D&D games.
Everyone runs their game differently, and I don't mean to even try and tell you this is the way to be a game master/DM/whatever term you use for the person who runs the game here.