What Is The Cause And Treatment For Social Anxiety?

I am facing this issue from school , everytime when some one ask me to read in front of class or talk in stage even about the known thing i get fear and i don't know whats happening around me my voice will start shivering , i feel like i performed very poor,feel very bad in front of others .I avoided all the stage talking in school , college etc. even when i walk in front of a big crowd i feel fear and my mind wont think. after completing my degree i went to many interviews the moment they call my name the fear starts till the interview gets over , i missed lot of jobs because of this . Now i have a job but in my job i need to give more presentations ,client meetings etc but when the meeting start i will get fear inside and perform poorly will not talk what i thought,but instead will blabber something, this issue makes me sad all the time , now i am not able to concentrate properly, forgetting everything. Please suggest me whats the some solution for this.