Speeches are about HOW they're presented. Excellent content may score you a B range mark (ie language appropriate to audience, textual references, thesis). But it's the oral techniques employed to capture and keep the marker's attention that get you the high range A marks.
So, in saying that, make sure you get over the freaking nerves, look people in the eye, talk "normally" and be interesting.
It's all about interest. Keep it, and you're set.
EDIT: have realised you were asking about structure.
Structure a speech like an essay, only with easier to understand language and more to-the-point arguments. Make sure the examples used are blatant.