All of my GMing rules posted so far are collected here.
Rule #2. You are not here to tell your story. You are here to enable the players to tell their stories.
“Game Master”, usually abbreviated “GM”, is a generic term. Some games actually use this term, while others choose something more idiosyncratic – “Dungeon Master” or “Keeper” or “Referee”.
A few games – especially games that came out of White Wolf – use the term “Storyteller”. And I cringe every time I hear it, because it puts exactly the wrong perspective on the role.
A GM isn’t a story-teller. They’re a story-enabler. They’re a story-facilitator. They’re a story-cultivator.
As a GM, you’re here to create a space, where a whole spectrum of stories are possible. You’ll set most of the parameters for that spectrum – through the setting you select or create, the NPCs you employ, the obstacles and objectives you put in place. But the unpredictable back-and-forth between you and the PCs is what determines, in the moment, what story actually happens.
If you have a particular story you want to tell, awesome! Go write fiction. If you want to GM, you need to abandon the idea of telling a particular story.
Because the PCs are going to do something else. Whatever particular story you have planned, the PCs are going to do something else. And maybe it’ll only diverge a little from your prepared story, or maybe it’ll have nothing to do with your prepared story.
When that happens, they’re not “messing up your story”. They’re telling a story that isn’t the one you prepared, and they might be playing a game that isn’t the game you wanted to be playing. In which case you maybe need to have a chat with your players about what kind of game you and they want to be playing, so that you can find the overlap and go to there.
Chat with your players about what they and their characters want. Find or create some neat toys – setting, characters, treasures, monsters, wonders – that you’re going to enjoy playing with, that fit with those desires. Then turn the players loose among these toys and see what stories they create.