pick something that your interested in, something your passionate about, music? art? video games? a film? if you choose a person try not to talk about thier education and when they were born etc, talk about why you like them and what they achieved, it its an artist, what the reasons or messages are behind thier work. dont be too complicated or complex, although your probably being marked on your knowledge if you go deeply into the physics of... i dont know, microwaves, people will get bired very quickly because they wont have any idea what your talking about!!!