It's easier to begin with the body or middle of your speech because this is where you express your main points or ideas of your chosen theme. You will add the conclusion and beginning later.
Choose 3 main points fitting your audience, purpose and theme. Start with the most important first. For each point you make give one or two examples to illustrate it. When you give your speech you'll link the points with transitions or bridges to get smoothly from one idea to the next.