My advice: Practice and preparation gets you a long way.
Read about rhetorics. Not only rhetorical devices but about different models of approach. I dearly recommend the quite short but oh-so-good "Ad Herennium".
After writing your speech, try to learn it by heart. There are numerous methods and it's not as daunting as it may seem.
Rehearse speaking to yourself, perhaps in front of a mirror. If possible, record yourself with a camera. Watch it and find where you can improve. Try rehearsing with a test audience, like a loved one or a friend.
When the time comes, don't be afraid to improvise. And most important: Relax. It's going to be fine.