Don't take the topic so seriously. For mine, I had to talk about a time when I had to show resilience. In my class we had things ranging from exams and sports days to being at funerals. Don't force yourself into something you find hard to talk about, and even if you find it interesting, don't talk about something that you'll find difficult to make cohesive.
If you are determined about talking about racism and world peace, then talk about how racism is opposing world peace and how removing racism from society would better the world. Or, you could talk about how if you ruled the world there would be peace because you think that disunity is the cause of all current and past wars, so you'd unite the world under one flag, with one currency etc..
As for general tips, speak loud and clear, try not to depend on notes (I personally forgot mine on the day, so I improvised and got 14/15) although if you need to look at them, do so.