Five minutes is not a really long time to speak, so obviously you are not going to want to choose the subject of nuclear sciences. If you have been given any guidelines at all from your instructor, look at those first. Whatever you choose, you don't want to find out later that you did not meet simple requirements.
If you have an open slate or the ability to talk about anything you choose, then the easiest thing to do is to give an account of something that recently happened to you. Perhaps it could be a retelling of an event you witnessed.
Another thing to do is to investigate short speech topics. There are several sources for searching, because it is a common assignment in speech classes.