The subject of your speech depends on three things, your audience, your own areas of interest or expertise, and the purpose or situation of the speech. Instructors also have different expectations based on the grade level of the class.
For an elementary school speech, something simple and personal would work. For example, you could talk about why you want a puppy or which sports you enjoy playing and why.
If you are in middle school or high school, you want to shift from purely personal speaking to something somewhat more informative for your audience. Instead of talking about why you want a puppy or kitten, you could discuss what is involved in caring for some type of pet, or rather than discussing why you like a sport, you might talk about the sort of commitment involved in being part of a team.
At the university level, you should move towards a short argumentative or persuasive topic, with a narrow focus. For example, you might talk about why it is better to adopt a pet from a shelter than to buy one, or why your town needs a dog park. Another possibility is to think of a persuasive speech advocating some small project which would improve some aspect of your campus, perhaps a way to donate uneaten food from the dining halls to homeless shelters or a way to offer healthier eating options or more convenient fitness options for students.
Avoid strongly politicized issues such as abortion, capital punishment, immigration, or gun control, as no matter what position you take on such issues, you are bound to offend someone.