Okay so there isn't really anything to say about confidence that hasn't already been said, but I'm going to share what I have found helpful for me.
1. Don't be worried that your not confident, most people aren't, they're just as nervous as you. So if your nervous to approach someone just remember that they won't even notice what you say or do because they are usually focusing so much on what they are going to say that they won't be concentrating on what you say.
2. Smile and laugh! If you look like you're happy people assume you are, and will assume you are confident. It is also important to be able to laugh at yourself if you make mistakes or fall over etc. Have fun and don't take life too seriously.
3. Know yourself and learn to love yourself. As cheesy as this sounds when you know what you're good at and can embrace it then people will love you for you!
4. Speak confidently. If you pretend you know what you're on about most people will believe you know your stuff. It sounds stupid but if you say something with enough conviction it will make you more comfortable with yourself and your opinions.
5. Finally, don't worry about not being confident most people struggle with confidence issues and as long as you have enough to feel happy with yourself that's fine. There's a fine line between confidence and arrogance
Good luck and have fun