How to be composed in front of people?

every time i am put in front of my classmates and required to do representation or some talkings,i inevitably feel nervous and my heart beats really fast.what's worse,my voice trembles quite obviously and i speak quite fast and incoherently. This annoys a lot and makes me feel akward and childish. I feel terribly out of sorts after answering question and cannot calm down myself. when i was interviewed in the expo volunteer program,i show too much nervousness and was refused to be a volunteer. i cannot figure out what should i do to change this "symptom".