You need to find a topic/subject that is of interest to you and it will make writing and presenting your speech much easier.
Back when I was in college, they made you give speeches in most of your classes and the only one I can really remember is one I did on Anti-Vivisection. If you don't know what Vivisection is, it is the testing of products on animals. I'm not a fanatic, but I am very against it, it is cruel and not necessarily accurate, but because I was/am so against it, it was easy for me to come up with alternatives, to be persuasive. So find something you are interested in or into, it will be easier.