So if you're speaking for 20 minutes, plan on preparing for at least an hour. Technical subjects may require additional planning time. Using relevant statistics or studies strengthens the force of your argument. Avoid reciting too many numbers, though. You don't want to numb the brains of your audience.
Thorough preparation is a great solution to public-speaking panic.
What's more, there's no English teacher who'll grade down for punctuation or spelling errors. You will too. Find a place where you can work without distractions. If your home is noisy, try the quiet study room in your local public library. One focused hour is worth three distracted ones.