You prepare situations, not scripts. A situation survives contact with the players; a script does not.
Keep it short. A handful of places, sketched enough that you could describe any of them cold. The people in each one, and what each of them wants — that is usually all you need to improvise the rest. One or two encounters you can see coming, so you are not building one from nothing at the table. A couple of names, held in reserve, to hand to whichever passerby the players decide to talk to. That is the whole list, and it fits on an index card.
Do not prepare an outcome. Do not prepare the one route between two places, or a scene that only works if the players have already made a particular choice. The moment you have written down what happens, you have also written down what you need the players to do to get there, and that is the job turning into steering. Players notice steering. They resent it, even when they cannot say why.
Prep what your group actually engages with. A dungeon full of traps is wasted work on a party that would rather talk its way past the guard than fight him, and a week of faction politics is wasted on a party that just wants to hit things. Watch what your table leans into, and prepare more of that.
Start small. Start in the middle of something already happening, rather than at a beginning that needs explaining. Give the party a reason to already be together, so the first ten minutes are not spent inventing one.
The conversation about tone and limits — what kind of story this is, what nobody wants to see at the table — belongs before any of this, and it is covered in the Introduction. Have it first.
Once you are ready to sit down and run what you have prepared, read Setting Difficulty. It answers the question every new DM asks before the first roll: how hard should this be.