Since the Homework Helpers can't tell you what to write on, here are some suggestions for coming up with a good topic. First, choose a subject that interests you. Let's say you like dogs. Then try to narrow the subject down to something you can write about knowledgeably. Now come up with a statement about your topic.
Beagles are the best breed of dog.
Wrap it up. Write a brief conclusion that summarizes the points you have made. Clearly, beagles are best because they're smart and they're just the right size.