depressed? self-esteem problems.do i need help? ineed answers !

im 16 years old, i dont have high self-esteem or confidence, never have really, and i dont actually know why. i find myself locking me in my room and crying, feeling lonely or sad ..feeling not good enough .out of no where , maybe something triggers it ?....i dont know whats wrong with me or why i am this way..im sure theres a reason but i cant understand it. what should i do? is there something really wrong ? i feel confused and like i need help but im not sure and im too scared to go ask for help other than this kind of thing i guess...until i know whats going on.