In short, gather more information than you'll possibly be able to use in your speech. The best speakers make their presentations sound spontaneous and conversational even when they are memorized. The way they do this is to learn the speech in outline form, instead of word for word. Your outline should contain the Opening, the Message and the Wrap up. Your opening remarks set the tone of the whole presentation.
Audiences make up their minds very quickly.
The purpose of your opening is to grab attention. We must assume that our audience is generally as busy and preoccupied as we are. So we first need to get their attention with a question, 'grabber' words, humor or an interesting visual. Using a question as an opener causes the listener to stop and think. Once you have their attention, your message can help them answer the question.