You don't have to forge the whole speech right now. Just sketch it out. Turn it into a speech Now that you're got a pile of verbiage, it's time to turn it into a speech. Go through it with a hard eye, picking out the gems and discarding the dross. Then arrange them in a structure that suits both the topic and your perception of the audience.
Repetition is essential for retention of a message.
With regards to voice, let your personality manifest itself on paper. When you finally speak, a comforable style will provide you a more relaxed environment to manifest your own personality. Estimate the time To best approximate how long your speech will be, simply read aloud three pages of written material at normal speed. The first objective is to get your audience's attention. Make your entrance fit your personal style, staying clear of a gimmick unless it really works for you.