It really depends on what your topic is. But here are some basics.
You really want to grab and hold your audience's attention. There are several good ways to do this.
Use an anecdote--that's a short little story that illustrates your point. If you were speaking about the negative influence of sports on children, you might tell a story of a child who became depressed due to parental pressure to perform well.
Give a convincing list of examples--this is a bit like an anecdote except you don't develop the examples into a story.
Use startling statistics. If your subject lends itself to statistical support, give some numbers that help make your point.
Ask the audience questions. Audience's love to be involved. Ask them questions, but make sure they are questions that you know the answers to.
Have a visual aid. Audiences find it much easier to follow a talk when you have something they can look at. Make sure it is visible from every part of the room. You might even include a handout.
You might start by asking your audience a rhetorical question (a question that isn't supposed to be answered at that time) that the rest of your speech answers. Then ask the question again at the end. It's a nice framing technique that emphasizes what your audience has learned. You can use a rhetorical question at any point, it doesn't have to be at the beginning.
Above all, practice out loud often. Time your speech so that you know you will meet the time limit. Make sure that you have an audience, even it it's just one person, for some of your practice speeches.
This is a good question. Since, eight minutes is not a long time, you would need to get to the point pretty soon. Here are some suggestions.
First, I would give the main topic of the talk right away without any delays. I would say something like, "today, I want to talk about the importance of ____."
Second, I would give a road map of why I thought this topic was worth their time, even if it is only eight minutes. So, for example, if the topic is on the topic of the environment, then I should give a few reasons why this topic was important to them. In short, all good speeches show right in the beginning the relevancy to people.
Third, it would be important to have a memorable closing statement. Keep in mind that this is the last point they will remember.