A State of Mind

That is really what "confidence" comes down to.  There are hundreds of websites on the internet that go into different points that one can do in order to become more confident.  However, at the end of the day, it all comes down to how you perceive yourself.  It's all in your head.  You can be the best in the world at something, with all of the tools and resources that you need, and still lack confidence if you don't believe in yourself.  I just want to list a few of the bullet points that seem to be repeated among various sources.

• Appearance - Dressing well and having good posture goes a long way.  Having the assurance of looking presentable is reflected in your daily routine.
• Don't dwell on failures - Part of this includes aiming for perfection.  We are not robots.  Look instead at your achievements and learn from past mistakes.
• Smile! - It really makes a difference.  There is a psychological process that is involved when you smile.
• Act - Face your fear without procrastinating.  Courage is not obtained before doing the thing you are afraid of.  Progressive desensitization and learning from mistakes will help overcome that fear.  The more you do something the less scary it seems and the more natural it feels.
• Don't compare yourself to others - Be yourself.  Embrace your uniqueness and create your own path to success in what you want in life.

It takes time to build confidence but it can always be done.  Be proud and succeed.

P.S. This one really stood out and should be thought about next time a friend gives you a compliment.