Self Confidence!

Self-confidence is the confidence one has in oneself, one’s knowledge, and one’s abilities. It is the confidence of the type: "I can do this". " I have the ability to do this". Self-confidence is the one thing that is much more important than many other abilities.Without this man's knowledge is not of any use.

According to me it is one of the most important thing that a man can have.
But for those who dont have so much self confidence.
One of the best example in which there is lack of confidence is me.
I know this is my negative point.But i want to be self confident like others!
I have 0% self confidence.
If i know something then because of lack of confidence i dont speak it and if it is correct then afterward i lament that why not i gave that answer even knowing that thing.
How this can be achieved? Can anyone help me?