I am 16, little, looking like 13, not good in sports or anything alike. I recently found that I lack any confidence. Even if I was good looking, I think I would still find myself to be confident around people. I was thinking of working out, but I don't think that will help too much.
I'll be 20 in July. I look like I'm 12. So...congrats, you look older than me. But the thing of the matter is...if you don't give a damn about it, it's much better. You'll grow into your looks. One day, you'll like looking 3 years younger. My dad is 57 and looks 29. So...you can imagine how it was for him growing up. But do you know how awesome he feels now? He has shaggy pitch black hair, all his teeth, no beer belly, he's fit, and active, and enjoys going out fishing and mountain climbing. All his 50-60 year old friends are fat and gray and can't do anything because their bodies are old.
So...shush your face. You look 13, not 12 like me. And in the future, it'll be better. So fake that confidence until you're actually confident. It works. It's all psychological. So do that. Ready? Go.
The beauty of our world is that everyone is attractive to someone for something. If your not fit or active, be dark and mysterious. If your not a social butterfly, be reserved and deep. There is no "popular" mold to fit in, the answer is to be comfortable in your own skin. Confidence will come with understanding and acceptance of who you are. Don't focus on what you're bad at, focus on what you accel at. Can't throw a football? Cool neither can I. But I have a passion for music and through that, I've gained many friends and girls. My friend never fit in at school, he was tall and skinny and loved black. But now he's a tattoo artist who expresses his deepest thoughts through his work and this guy is more popular than any jock I've ever seen. It's not about what you are, It's about who you are and how you perceive yourself. Keep this saying in mind "I think, Therefore I am". Just do what you do, and best the best that you can be at whatever it is you do. People will love you for that.
I'm 18; when I was 16 I'd never done any sorts of sports, at all, full stop. I was (actually, still am) terrible at hand-eye coordination and all sorts so just thought "screw it". However I was really nervous and lacked a lot of self-confidence. I never considered the gym, at all; then a friend of mine made me start going and I got into it, and let me tell you, it transforms your confidence tenfold. I'd thoroughly recommend it. Find a friend to motivate you and you won't recognise yourself in a year.
Most 16 year olds have no confidence. You have not lived long enough to develop any, and you haven't thrown yourself into any situations which will help it grow.
For now, don't worry about it. Throw yourself into new situations, challenge yourself socially, fight any fear you have which prevents you from doing stuff. You'll fail at some things, you'll succeed at others, but mostly what you'll do is realize that failing at something is *nothing* to get upset over.
That is going to be the bedrock of the confidence you build, because you will be learning that no matter what you do or try to do, you will have the tools and experience to deal with *any* outcome.