Pick something you feel strongly about. If you don't feel strongly about your topic, how are you going to persuade the audience to feel the same way? Students sometimes say, "But I don't feel strongly about anything!" Yes you do. Sit down with a piece of paper and brainstorm. If you like baseball, give a speech for or against salary caps. If you like rap music, give a speech on why rap music is not as violent as many people think. You get the idea.
Pick a current event. Having trouble thinking of a topic? Go read a newspaper. What's going on in the world? If there's an election, endorse a candidate or a ballot referendum.
Pick a campus or local issue. Are there controversial issues around campus? Are there controversial issues in your college town? These topics will be very relevant to your audience members.
Pick an issue of interest to the audience. Give a speech about cell phones, or music downloads, or tuition hikes, or something the audience cares about. If they don't care about your issue, they won't be persuaded.
Pick a smaller part of a big issue. Don't try to change people's mind about a huge issue in your short speech, because you can't. Think you can change your classmates' mind about abortion in a 6-8 minute speech? Of course not. However, you might change their minds about a portion of this issue, like parental notification laws.