Preparing a speech is very much like preparing a document that is written. However, there are some things that are different. Like anything written, you must know your PURPOSE and AUDIENCE. These two factors (along with the time allocated) dictate the content. If your purpose is to persuade and your audience is peers and a teacher, you will write differently than if your purpose is to amuse and your audience is work colleagues.
You need to know the content. Is it factual? Is it statistical? Is it full of jokes? Are they appropriate? You will probably need to do some research. You must find quotes. Quotes are compulsory. They are usually from experts, clever, short and sharp. As it's a speech, build in the word "you" as much as you can as it's all about them. As it's a speech, ask lots of rhetorical questions. As it's a speech, paint lots of pictures with stories and metaphors. As it's a speech use heaps of alliteration, such as "Station, nation, creation are examples of alliteration" and, of course, use much repetition. It's effective in a speech and not suitable in most written texts.
The structure of a speech is: intro, body and conclusion. Do not worry about how you will start. It needs to be sizzling. Inspiration may come at any point. Let them know your summary and then launch into your first point. Follow up with some evidence or elaboration and then, if suitable reinforce the point made. The next point should be linked to the first. You can use "Firstly", "Secondly". However, it's a little boring, but it lets your audience know where you are going. The conclusion should be signaled with specific words, "In Conclusion." or "Let me finish with." or "Before I close." Please do not add "Thank you." It is polite and boring and not memorable.
Just as you need to start with impact: a story, a stat, a quote, an image, so you need to finish with even more impact. You may call for action, if it's appropriate. The best finish is with a quote and let the quote be the last thing you say.
Once have finished the preparation you start to practise. Read my next blog on practise. you will be glad you did.