basically, practice makes you confident. which completely makes sense. the more experience you have with something, the better you become. public speaking can be seen as a conversation. bring up topics that relate to the audience, allowing them to be continuously engaged. make your everyday conversations intentional practice for when you find yourself standing in front a crowd of people.
A shy man no doubt dreads the notice of strangers, but can hardly be said to be afraid of them. He may be as bold as a hero in battle, and yet have no self-confidence about trifles in the presence of strangers. CHARLES DARWIN