Establish details, describe
All the ideas and visions in your head don’t exist in the fiction of the game until you share them, describe them, and detail them. The first session is the time to establish the basics of what things look like, who’s in charge, what they wear, what the world is like, what the immediate location is like. Encourage everyone to contribute by asking the players questions-use what the players give you. Let everyone describe what they please but keep it brief enough to expand on later. Let a unique detail or two to make a description really stand out as real.
If you ever find yourself at a loss, pause for a second and ask a question. Ask one character a question about another. When a character does something, ask how a different character feels or reacts. Questions will power your game and make it feel real and exciting. Use the answers you find to fill in what might happen next.
This is one of your principles, but it’s especially true during the first session. Every blank is another cool thing waiting to happen; leave yourself a stock of them.
Look for interesting facts
There are some ideas that, when you hear them, just jump out at you. When you hear one of those ideas, just write it down. When a player mentions the Duke of Sorrows being the demon he bargained with, note it. That little fact is the seed for a whole world.
Help the players understand the moves
You’ve already read the game, the players may not have, so it’s up to you to help them if they need it. The fact is, they likely won’t need it much. All they have to do is describe what their character does, the rules take care of the rest.
The one place they may need some help is remembering the triggers for the moves. Keep an ear out for actions that trigger moves, like attacking in melee or consulting their knowledge. After a few moves the players will likely remember them on their own.
Give each adventurer a chance to shine
As a fan of the heroes (remember your agenda?) you want to see them do what they do best. Give them a chance at this, not by tailoring every room to their skills, but by portraying a fantastic world (agenda again) where there are many solutions to every challenge.
NPCs bring the world to life. If every monster does nothing more than attack and every blacksmith sets out their wares for simple payment the world is dead. Instead give your characters, especially those that the players show an interest in, life (principles, remember?). Introduce NPCs but don’t protect them. The recently deceased Lord of Goblins is just as useful for future adventures as the one who’s still alive.