Every story has a beginning and end, and the storyteller gets to choose them. There's more to that than you think. When I was a magazine editor we let our writers just type away and have a good time. When they turned in the manuscript, we would have to find the true beginning. Usually the writer had to get a lot off his chest before getting to the real story.
I once worked in the New Orleans bureau of The Associated Press. The joke there was that when we were picking up a local newspaper story for distribution on our wires, we'd immediately go the seventh graf to find the real lead.
So don't assume that, for instance, your personal story begins when you were born. Of course it does, but not when you're telling someone else. It must begin at a point that will immediately engage your audience. Your story will end when you die, but that's for you and not your audience, so you also have to decide where to stop.
Chris Anderson, a former journalist, is the curator of the TED Conference, which features the world's most important speakers. Anderson advises this:
If you frame the talk as a journey, the biggest decisions are figuring out where to start and where to end. To find the right place to start, consider what people in the audience already know about your subject—and how much they care about it. If you assume they have more knowledge or interest than they do, or if you start using jargon or get too technical, you’ll lose them. The most engaging speakers do a superb job of very quickly introducing the topic, explaining why they care so deeply about it, and convincing the audience members that they should, too.
Many of the best talks have a narrative structure that loosely follows a detective story. The speaker starts out by presenting a problem and then describes the search for a solution. There’s an “aha” moment, and the audience’s perspective shifts in a meaningful way.
If a talk fails, it’s almost always because the speaker didn’t frame it correctly, misjudged the audience’s level of interest, or neglected to tell a story. Even if the topic is important, random pontification without narrative is always deeply unsatisfying. There’s no progression, and you don’t feel that you’re learning.Ah, the aha! moment. That's where you pause for effect and end it. Why step on the punch line? You're done your job. Get outta Dodge.
From Wikipedia: The Eureka effect, also known as the aha! effect, refers to the common human experience of suddenly understanding a previously incomprehensible problem or concept. Unfortunately, the writer of the Wikipedia entry says that, "It is difficult to predict under what circumstances one can predict an Aha! moment."
You could study your audience's faces, I suppose. You might learn over time as you hone your presentation. Often a powerful anecdote illustrating your thesis can be used to generate the epiphany you desire. Achieving this, of course, forces you to understand the one thing you want to convey in your speech. Knowing that will help you know where to start and where to end.