Back Posture is hurting me at teenage.

Hi i am 17 year old and have been using computers since age of 11. I had bit lower back pains but never bothered to inquire about it. I have sat most of my life and now i am 17 since 2 years my back strength is too weak i cant even stand erected for long time i have to lie down or sit. My weight has also increased and problems have become worse. Daily in morning my shoulder neck and back is paining at least for fifteen minutes and my chest feels short of breath. My posture has become very odd (I have added pictures). This is indirectly affecting my work and studies a lot my self confidence has also decreased. I need proper guidance for improving my posture and strengthening my back. Thanks for help in advance. I really appreciate everyone's help. Link for my body posture - https://i.stack.imgur.com/FiQYI.jpg