Confidence is, first and foremost, the reflection of a positive self image. To have confidence in yourself, you have to like yourself. To inspire confidence in others, you have to learn to have confidence in yourself.
To convince others, you first have to be convinced yourself. Why do charismatic persons have absolutely no trouble finding people to rally behind them? Because they are already convinced that what they're doing is right.
By learning to appreciate yourself, by adopting the attitude that success will come to you as your right, since you are just as intelligent, competent and careful as anyone else, you increase your potential for success astronomically.