If you want to become famous at that age you need to try starting from the beginning. Build up your confidence by doing lots of school talent shows and music evenings and maybe some at holidays. Then maybe start to perform at jam nights (for singing) if you go with an adult and listen to other peoples views and not to take it the wrong way. Feedback is very important especially if you want to go on to being famous so you can improve. Then if you do this for a long time maybe you should try and audition for something bigger - to do well known plays at west ends and maybe try to get in an academy because that will really help you with your career.
