I want to be more confident, but I don't know how?

I'm really quiet, and I don't talk much. I dress plainly to not attract attention, and I am not confident in anything I do. I feel like I have really bad self-esteem. So many other people I meet seem so confident and happy. I want that, but I really can't figure out how to change myself and my mindset. I feel like I'm not worth that sometimes.