I'm not a very self confident person. This is used as self assurance in one's judgement, ability and power.
I'm just the opposite and sometimes it gets me into trouble! I should learn when to keep my mouth shut, for example.
People increase self confidence from experiences of having mastered many different activities
That's a very good way of gaining confidence. Have you tried many things to find something you enjoy and are good at?
Not really. I need to though. It will increase my feelings that in the future I will be able to accomplish what I wish to do. 
You just have to believe in yourself. Start with small things and build up. Before you know it you'll be an expert in whatever you choose to learn.
Self confidence is different than self efficacy, which is defined as a belief in ones ability to succeed in specific situations.