Playing Pathfinder for the first time can be a daunting task, especially for a new Dungeon Master. To make it easier to get started quickly, there are fortunately pre-made campaigns (also called Adventure Paths), where you don't have to create monsters, environments, and encounters yourself and where you know that your adventure has been thoroughly playtested.
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The most important thing is not to know all the rules or have perfect character voices. The most important thing you can prepare for your first game is an adventure that ensures your players have fun.
You should read the entire adventure from cover to cover before running it. If you really want to give your players a first-class experience, then practice speaking like some of the NPCs or monsters in your free time. You should also make sure to have read the monster stat blocks and dungeon areas in the adventure, so nothing in the adventure text catches you off guard. I guarantee you that your players will do things you don't expect, so be sure you at least know how the adventure is supposed to work, so you can improvise more easily when things inevitably go off track.
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