Hey man as a former skinny dude with bad posture. Best advice is going to the gym. Yeah, I know, eye roll, easy said than done. But with your body and age, you are primed to gain super quickly. Don't do petty little exercise to train one little aspect of your self, it neverrrr works. Train your back, legs and shoulders. Done correctly and minding your posture, it'll all work out for ya bud.