Simply put the Storyteller has a story to tell. This starts with deciding how you plan to introduce your players to the world, what that world looks and feels like, how your players will find each other and the options that will be set before them for adventure. The Storyteller should also loosely define how long the particular campaign will last, how many sessions it will take to cover and what can or should happen during these sessions so that progress is felt and a clear ending of some kind is experienced.
There are many books on the subject of being a Storyteller, or Dungeon Master (DM) as some games define it and even more opinions on how one should go about the job; from planning every single step to creating an outline and letting the players drive. Truth be told every Storyteller is going to be different and there is no magic method that will work for everyone. Finding a style that works for you will come naturally as you begin to construct the world and the events that shape it.
Keep in mind that while the Storyteller is responsible for setting the initial tone, you are working with players who are people too and their characters are very likely to want to go the opposite direction you had planned, skip the dungeon that you spend a week designing and ignore the dialogue you spent days writing to jump straight into combat instead.
The best advice I can offer you, future Storyteller, is to be prepared for anything. Because if your players are anything like mine, they only certainty you will experience is uncertainty.