Practice Public Speaking

Speaking in public is a terrifying and rewarding experience that becomes less terrifying and more rewarding each time you do it. As a young person, opportunities to speak might include class presentations, accepting awards, running for school or club offices, or speaking to administrators about your needs and views. 

There are countless websites and groups, like Toastmasters, dedicated to teaching you how to speak in public, so I won't go into detail here.  Instead, I will encourage you to look at public speaking opportunities as a gift that you should take, and not a chore to avoid.