Posture is a good place to start. I had terrible posture in middle school and my dad told me that men need to have good posture to be respected. Your posture affects the way people see you and how you see yourself.

Secondly, realizing that people don't care as much as you think. I actually learned this one on my own in high school. Everyone's overly concerned about what everybody else thinks of them. Once you realize that almost everybody else has the same insecurities as you, it's easy to just be okay with yourself and focus on your good qualities rather than the bad.

Lastly, try thinking of something that you're good at. Every single person that I've ever met was good at something whether it be cooking, public speaking, sports, studies, etc. Hell, even being good looking can fit in this category. Of course you're not gonna be the best at something or the best looking person, but being above average in even the most menial of things allows you to have something to be confident about.