This statement seems tautological in nature but the key to self esteem/confidence is to TRULY have esteem and confidence in self. Whatever may be preventing you from having such confidence is another story but when one truly has confidence in self, it radiates to everyone around them and it is known, sometimes without a word being said. There may be genetic advantages such as attractiveness and height that can help with self esteem, but confidence is not exclusive to those traits. I have seen (forgive my vanity) short, ugly guys pick up attractive girls solely due to their confidence.
I can't tell you how to be confident because everyone has different complexes, life experiences that may be affecting their confidence. However, a good start to building confidence is always with a confident image. Walk straight and upright, keep eyes level, never down... straight shoulders. It never crosses most people's mind that the way you walk tells almost everything about someone's personality...