There are a couple of ways you can GM a Pen and Paper RPG.
A prepared adventure is one where everything has been worked out already. Usually down to the sequence of encounters the characters will experience, the events they will go through and how it will end.
It makes it a simple progression usually. No matter what the players do, they will pretty much always end up following your adventure's series of steps. And that works fine much of the time.
The great advantage of a prepared adventure is that everything is already set up for you. You have no mysteries and nothing that hasn't been worked out in advance. You're not going to wonder "what do I do now?"
But there are a couple of problems with the prepared adventure.
The main problem is that there is no real self determinism of the players. No matter what they do, the outcome will basically remain the same. The only choices they truly have are how to overcome what has been put up against them.
But if you've ever run a game before, you'll realise that players never do what you expect them to do. And that quite often leaves the adventure seeming a little off balance. The monsters are still waiting in a certain room, even though logically they would definitely have heard that the players were there, and probably either hidden or come looking for them!
That brings us to our second type of adventure:
Story Adventures are those that are centred around the world setting and a situation. Essentially, you as the GM work out what is going on the players' world, and then present them with a situation. You might as well say that you create the world and the situation, then plonk the characters in the middle of it and see what happens!
This is a very free type of game, giving the players great scope of potential action. Literally, they could do anything and the GM will simply work out how that affects the situation.
For instance, if you decided that the situation was that they ended up in a turf war, you would present them with something that brought them into the situation, like a violent encounter with a rival gang member. Whether they maim, kill, disable or flee from this gang member would then determine what the rival gang would end up doing in revenge.
You would then work out whether the gang could track them down (making certain rolls - keep it honest! Remember, the pc's could just get away!). When they track them down, you would hopefully already have created some principal characters which could go toe-to-toe with the players.
And then from that encounter you would see another reaction. Perhaps the gang would become afraid of them. Or would try to placate them. Or another gang would want them to join up.
Essentially, this second method still requires preparation! But not like the painstaking step-by-step first method! You just need to prepare key areas, key NPC's, and know your world, so you can quickly work out reactions (and for that you could roll a die).