Kristoffer,
Dungeon World can work in different ways: some people *like* just making things up willy-nilly. However, the basic idea which most people prefer to that kind of wild, open-ended creation is this:
* You can ask questions of the *characters*, but not the *players*.
This means that, before you ask a question, make sure you phrase it in a way that the character has a chance to answer.
For example, if the heroes stumble into an underground tomb, and they look to you and say, "What do we find inside?"... it would be pretty awkward for you to ask them: "Well, tell me, what do you find?" They'll likely give you some blank stares. And rightfully so, because isn't it your job to tell them what's there, and their job to discover it?
However, when you don't have an idea ready, you can leverage their creativity. Re-phrase the question as something the characters could answer:
* "This is the tomb of an the ancient Crimson Order [something you established earlier in the campaign]. Kyrion, what's the wildest story you heard as a child about the Order's creations?"
* "When you wandered into this region, it was because you sought treasure. What's the most famous treasure that's rumoured to have been lost in these lands?"
* "Ragnar, you have done a great deal of reading about the curses of the Elder Gods. Want to share your findings with the group before you enter?" If they do, have them roll to Spout Lore.
This works well because it's familiar to the players (they're just playing their characters in response) and because it preserves the suspense of discovery: they'll throw you ideas but they still don't know what exactly they're facing. Sometimes you can have their guesses be exactly right (and they'll cheer!), sometimes it will be a nasty surprise instead (but make sure you give them a chance to avoid it, so they don't feel like you trapped them by intentionally going against what they just said), but most often it will be something like what they said, but with a twist of your own. Here's where you make a move and bring forward a threat!
John Harper has a good little essay about this, though it's aimed more at designers:
http://mightyatom.blogspot.ca/2010/10/apocalypse-world-crossing-line.htmlFor example, let's say your players aren't quite on the same page as you about the game, or they don't totally trust you just yet. They grin and tell you, "Oh, yeah, sure, we heard that these tombs are supposed to contain incredible piles of gold, a powerful magic sword, and books of lore!" "Oh, right, and I also heard that the tombs were completely undefended and never had any traps!" "Yeah!"
Well, that's not great. But you can still use it:
"That's exactly right. You enter the tomb to find an enormous cavern. It looks like there was a battle here recently: the bodies of brigands line the floor from wall to wall, the stone is covered in dried blood. Whatever treasure was once here has been thoroughly looted. However, there's also something frightening: some of the bodies were sliced in half, as though by a great scythe, and the flesh around the edges has turned a otherworldly green. The wounds writhe with glowing maggots, about as long as your thumb."
You went with what they said - and the natural consequences! unguarded treasure won't be around for long - and added a twist to remind them that they said something about a powerful magic sword. Perhaps they'll stick around for a while, and those bodies will reanimate and attack them, or perhaps they will try to track down the victor of this particular fight to try to claim the treasure themselves. But maybe the Wizard will inspect the wounds and Spout Lore and you'll find out that it wasn't a magic sword at all but a dangerous, forbidden sorcery. Now you can work all those things into your Fronts, building naturally on what has taken place.
You'll now have dangerous bandits, armed with a magic sword and tons of gold they can use to hire a small army, or an outcast sorcerer who will cause havoc in the region, or some communicable otherworldly disease transmitted by glowing maggots.