I'd love to be able to public speak,but I get a bad stutter?

Hi,I'm 14 and would love to be able to go to drama or debating club and stand up for stuff i believe in,but when I go up to talk i get really freaked and my voice changes and I stutter. Most people are surprised,cause im quite spontaneous it's just when everyones watching and listening to me I can't!!! Any tips? PS I went once to debating and fot up to speak and cried. Yes,I know I'm useless :P