The body of the speech is why you are presenting a speech to an audience. The body of the speech will come primarily from your brain, that accumulation of experiences and thoughts. But to increase your effectiveness in public address you will need to spend considerable focus on your introduction.
An introduction is a multifaceted creation. It moves the members of the audience from where they are to where they must be to understand what you are going to say. It includes: 1) topic (central idea), 2) breakdown of the topic (main ideas), 3) who you are and why you can say what you're going to say, (credibility) and why and how this information will be important to them. Throughout this introduction you will be gaining their attention.
Introductions need to be short, clear and easy to follow and recall. Everything in the paragraph above can be said in a very short time. Details are eliminated and general statements will be used.
Introductions are used to tie your subject to the topic included in the body of your address. The audience then has an idea of what you are about to say and the order in which you will be saying it. In short, it quickly builds a structure which will allow them to attach information (data) to as you move through the thought process.
Introductions are so important it would probably benefit you and your audience if you actually wrote out the entire introduction word for word. That will help you to see the strengths and weaknesses of the introduction before you stand before your audience.
This same process will aid you in your conversational practices. You will correctly analyze what the other person is going to need to understand what you are about to say.