When beginning a speech with a story — one of my favorite techniques — it’s not necessary, helpful, or wise to say anything about it. Just begin telling it.
(As a rule: the less you have to explain a story, the better.)
When seguing into a story somewhere later in your speech, you may need to alert your audience that you’re shifting from cognitive content (exposition, explanations, evidence) to an imaginative element.
The easiest way to do so is by using one word: “Imagine.”
Say, “Imagine.” Pause. Then, without further elaboration, tell your story.
Check out How to Tell a Story in a Speech.