You have talked about the problem that is common in teen ages. It frequently happens when we are to face people we lose confidence and we start staring in the space, having our hands or feet trembled, voice stammered. These are the symbols of ill-confidence. Not let you know it (ill-confidence) continues till we have a good command over the topic on which we are presenting, we are doubtful about any content we are going to deliver, we fear that people would criticize, etc. The most important thing one should remember that the audience is ignorant to what he or she is delivering and he or she is the most sincere person of the audience. Never think that people are insulting you by smiling; instead realize that they are showing happiness what you are telling them. Always deliver the amount that you can easily take out, don't over-run.