Gather your main points and create a focus. Ask yourself what the main idea of your presentation is: the one or two things you want the audience to learn. Focus on these and structure your message around them.
Research the topic and create an outline. The outline should be based on your key points, with information to support them. Now that you've narrowed your focus, it's time to organize the information you will use to convey your message. Categorize your subject by breaking it down into smaller bits: pros, cons, history and function are all examples. Much will depend on the particular topic you are writing about. The idea is to present it in an organized, easy-to-understand manner.
Keep your audience in mind. Much depends on the age and demographic of the people you are addressing. For example, if your speech is for adolescents, you may want to include plenty of humor and anecdotal bits to support the points you are making, as attention spans run short with such groups, as a general rule.
Use a conversational tone when writing your speech. This will help keep the audience engaged and will prevent you to from sounding dry or boring.
Consider using the problem-solution method in your speech. This can work with a lot of topics. Address the problem or issue, and explain it detail. Then, follow up with the solution, how to remedy the issue.
Revise your drafts. Go back frequently to see if there is anything to add or omit from your speech. Many times, less is more, regardless of the group you are writing for. Also, be mindful of the speaker you are writing for if it is not you who is giving the speech. Know his or her strengths, weaknesses and tendencies with public speaking.