The most persuasive speeches often come from talking about something with which you are intimately familiar. If your options for a topic are wide open, I encourage you to speak about a social issue that is important to you. Perhaps this is something like bullying, a rule at school that many students disagree with, or the stress that comes from the high expectations of high school students.
Once you've come up with a basic idea for a topic, you need to gather samples of evidence to prove your point. In any social topic, especially one that is current and personal, you can use real life examples as effectively as research and statistics. In writing your speech, gather as many examples that you can that help make your point. Once you've gathered several, you should be able to group similar items together and a natural organization will begin to take shape.
Once you've organized and outlined the substance of your speech, the opening and closing lines should come very easily and naturally. If this is truly a topic that is important to you, your own passion and emotion can drive your hook and your final line.
Consider opening with a question, joke, startling statistic, a quote, or even a short but personal story. Then, close with whatever is the most important point you wish to leave with your audience at the end of the speech.