I used to be pretty withdrawn, shy, introverted, you name it. Then one morning during the summer before my Senior year in High School I woke up and decided I was sick of being entirely unknown and seen as just some quiet brainiac with marginal looks. Sure, the decision came one morning, but the changes are still happening 2 years since I've graduated. I love laughing at myself. I love laughing. I enjoy just hanging out with a bunch of good friends. Sure, I'm still a bit quiet around people I don't really know or when I just meet someone, but I now know that it's okay to open up. People are very often willing to accept you as atleast an acquaintence.
For me, it was a matter of determination. Just simply deciding that I wanted to change. But no one can give you an equation that will work no matter what. It is yet another one of life's lessons that you must learn on your own.