At the start of your talk, you have a big attention advantage. You've been formally and perhaps enthusiastically introduced; the men and women at your presentation are hoping it will be interesting, hoping they will get something out of it, and hoping it will not be a waste of their time. Your audience is in a state of expectation, and all you have to do is be reasonably confident, knowledgeable, and prepared at the beginning, and you'll have the audience on your side.
The opening is your appetizer; it is not meant to satisfy but to tempt, titillate, and arouse, and whet appetites for the next course. If you fail to get your audience's attention at the very beginning, it will take you at least three minutes to get it back, and people will already be less than excited about what is to come.
Compare these two openings:
"Um...hello, I'm your speaker, Debbie Massey, and I'm here to give some, or a few, clues on what foods to avoid so you can have less disease and less stress."
"Ladies and gentlemen: Would you like to add 20 quality years to your life? Then THINK before reaching for your salt shaker. I'm Debbie Massey, and I'm going to share with you 10 easy, proven steps to add those 20 years to your life."
Years ago, the great architect Frank Lloyd Wright gave a speech in Pittsburgh. His attention-getting opening was, "This is the ugliest city I have ever seen." Pittsburgh paid attention—to the opening and to the rest of the speech. In a recent survey, the city was ranked as one of the most desirable places to live in the United States. Wright knew not to start out with, "Good afternoon, it's a pleasure to be here," or with an irrelevant joke just for the sake of opening with humor. He came out swinging, and even if the people in the audience didn't agree with him, he had them listening.