There is one thing all self-confident people share: a good posture.
This is not something you notice consciously but something you know at a certain level.
Slumped shoulders, lethargic movements, head tilted with eyes looking down is the mark of an insecure person whereas the posture of a confident person is completely different.
Can you remember a time where you succeeded in something? A time when you were completely happy and satisfied what you have achieved, and people congratulated you.
Try to remember such a moment, even a small one.
Now try to remember what you felt and what your posture was.
Try to mimic it right now.
Stand up straight, keep your head up, relax your body but be firm in your movements, gaze confidently to things around you.
You will notice that you will immediately feel more confident and that is one of the things you can do to build self-confidence.
While you should primarily work on your belief system try to work on the posture also.
"Fake it till you make it" and imitate the posture of a confident person.
It does not stop with posture.
Pay attention to the way you speak.
Is your voice high pitched and fast? That`s a sign of insecurity.
Notice how really confident people talk.
Watch political personalities, actors or singers and try to emulate the way they do it.
Remember that what you communicate non-verbally is often more important and has greater resonance that what you say.
The first thing other people see is how you carry yourself and right there and there they can tell if you are confident or not.