The master of the short story, Edgar Allan Poe, stressed the singleness of effect. This element is paramount to the coherence of a short story. For, too often writers feel the need to embellish their narratives in an erroneous way of adding what amounts to a digression from the single theme.
I mention Poe because many of his stories begin in the manner that you first lines suggest. So, perhaps, you may wish to refer to his stories such as "The Tell-Tale Heart," "The Black Cat," and "The Cask of Amontillado" that immediately introduce the first-person narrator, albeit an unreliale narrator.
Here is another reference that may assist you: http://www.enotes.com/theory-short-fiction-salem/theory-short-fiction
To get started, you need to envision who is saying the quote and about what to whom. Is it bad employment news? Is it awkward romantic news? Is it unpleasant political news? Did someone get called back into the CIA after quitting to raise a family? Next, you have to know what the thing they can't say is. From there, you can consider the potential effect of the news on the listener and the motives of the speaker.
Every good story has a beginning, a middle, and an end. That sounds so simple, I know, but it's more difficult to do than it sounds. That means that after an expostion, in which you introduce the setting, characters, and basic situation of the story, there must be a dramatic moment of change which moves the plotline from a flat line to an arc moving up (rising action). This is a conflict of some kind. At the story's peak (climax, crisis), there must be a turning point or reversal which is a result or consequence of the conflict. The resolution (denouement) should tie up all the loose ends and provide a satisfying conclusion for your readers. Happy writing!
You can also start by defining your characters. Who said this line? Why did he say it? Write a list of descriptors of the type of character and situation that would cause this line to be said. If you are having trouble coming up with characters...spend some time people watching with pen and paper in hand. Write down observations like how people walk, talk, interact, move. Once you have figured out the history and biography of your main character, it may then be easier to create a story around him or her because you know the character so well.
Just as mentioned in the above post, you can always begin by answering the following questions:
Asking yourself these questions, is a great way to get material for writing. Beginning with a problem or a conflict is a good way to get started. Who are the characters. What is the problem? When did it happen? Where did it happen? Why did it happen? and How did it happen? After answering these questions, you will have a story.
You have a good beginning, and that is sometimes the hardest part. Consider who this person is. What is it that he is practicing? Why can't he say the words when the time comes? Is he nervous? Is there someone in the crowd making him nervous? Did he write the piece and he's nervous to perform it? These questions will allow you to build a character and plot around this beginning.