You are also in charge of the story itself. The players will start off knowing nothing about the story they are experiencing, and will learn about it as they go along. You, on the other hand, must be able to jump-start things from a dead stop. You must always keep an eye on the progress of the story and make sure that all of the players are enjoying themselves.
Indeed, your foremost duty is to invent a story, or at least the beginning of a story, and to simultaneously entrance and entertain both your players and yourself. However, you cannot simply create a story and tell it to an audience: it is your interaction with the players that creates a plot. A story is invented moment by moment, through the dynamics between player and Storyteller.
The foremost responsibility of a Storyteller is to make sure that a good story is being told; however, the Storyteller should never simply tell a story. The events and flow of the story are as much the responsibility of the players as the Storyteller. The primary duty of the Storyteller is to lead the story and to keep it moving briskly in the desired direction - or at least stop it from breaking down completely. Telling a story is more a matter of keeping up with the players, commenting and elaborating upon what their characters do and say, than it is of relating a narrative.
The most important thing when telling a story is to be ready to abandon your expectations. Never force the players to follow your premade plot; you must be willing to work with them. Try to anticipate what the characters will do and how this will affect the course of the story – you can't foresee everything, of course, but have a few contingency plans worked out in advance.
Before you begin to tell a story, be prepared. Don't have a plot already set, but have story hooks ready, and create as elaborate a setting as possible. This may sound obvious, but it can't be stressed enough. If you are using a premade story, read through the text of the story very thoroughly at least once before you present it to the players. Make sure you know when and where everything happens, who is where at what time, and so on. In particular, study the major antagonists, their personalities and motivations, and be sure that you can present them convincingly to the players.