Know your subject. Part of making yourself a comfortable and dynamic public speaker is to make sure you know what you're talking about and you know it well. Lacking knowledge can make you anxious and uncertain when you're speaking and that will come through to your audience.[1]
Preparation is key. Take your time when you are planning your speech to make sure that it flows naturally and logically. You'll also need to make sure that you know how you're coming across while giving the speech and heighten your good qualities while downplaying the less good qualities.
Even if public speaking is something like having to answer a question in class, you will still need to make sure that you know your subject. This can help you feel and present as more confident, which will make a good impression on your listeners.