It's hard to change one's character.
But one advice: Think positive.
When you're feeling positive & confident, it shows. The
cheerfulness you exhibit will attract people to chat with you and
know you better.
Soon, you won't find it much of a problem to be sociable.
Also, try not to judge. Each of us is different in our own way.
Accept people for who they are, and they will accept you for the
quiet character that you are inside.