Read your teacher's instructions very carefully. The instructions might include what type of topic you need to pick, how long the speech needs to be and what the purpose of the speech is. For example, the purpose might be to inform your audience or to persuade them.
Choose a topic. Narrowing down your topic will make writing the speech easier. For example, the solar system is a very broad informative speech topic, while "Why Pluto is not longer a planet" is a narrow topic.
Write the introduction for your speech. The introduction tells the audience what the speech is about and goes over the main points briefly.
Develop at least three major points for the speech. These points directly relate to your topic. For example, one point from the above example might talk about what information is used to categorize something as a planet in the solar system.
Add additional details to each point that back it up. These details should come from your research on the topic, based on legitimate sources. The NASA website, for example, would be a legitimate source to go to for information about Pluto.
Conclude your speech. The conclusion goes back over your main points to remind your audience of everything you've addressed.
Read the speech you've written out loud. This will show you if any of your sentences or points are not complete or sound awkward. Practice your speech until you're comfortable saying the whole thing out loud.