When it comes to being a confident speaker, thorough preparation is half of the battle. You can't just get by on a smile — you need to know what you are actually talking about! Do your research, study ahead, take notes, and make sure you know your subject inside and out. There's no such thing as too much preparation.
You'll find that the more secure you are with the content of your speech, the more confident and flexible you will be when you actually deliver it.