What and how much should i prepare

I've DMed a few of my own adventures, but when im preparing for them i tend to get too caught up in writing the story that when it comes time to play its almost entirely improvised. Which often ends up with me stalling while i think up an encounter or what an npc will say. What should i be writing down in preparation for the adventure. Also im going to be DMing princes of the apocalypse if anyone has any ideas for that campaign in particular