While others are recommending platforms to practice, here are my 2 cents. Most important asset for a public speaker is "clarity". Clarity of thoughts is a magnet for audience attention. Once you have an audience "connect", anything you say is gold. Clarity of thought can be improved by these two resource books:

1. Art of Argument - Giles St.Aubyn (ISBN 0800803698) 2. Influence - Robert Cialdini (006124189X)

After that, try reading some Bertrand Russell writings. See how your argument and persuasion quality jumps from above resources.