Proposition of fact...?

I think this is something that you've got to choose on your own, because the key to a good speech is convincing yourself that you're telling the truth. Once you've got that, convincing others is easy. In regard to your topic, choose something you find interesting, but also something you can prove reasonably in 3-5 minutes. Keep in mind, it doesn't have to be a world changing truth; think apples are better than oranges? If you can prove it in 5 minutes, your teacher will be impressed. I realize this isn't an answer, but I hope it helps.