This is going to sound INCREDIBLY strange, but trust me, there is science backing this up. Every day, for at least 2 minutes, stand with your feet apart, back straight, chest out, and your hands on your hips. Studies have shown that this increases confidence. (I like to pretend I'm a super hero. xD) Exercise also increases confidence, as does taking a martial art. And mentally, it's just important to accept yourself for exactly who you are and not care what people think. If something makes you happy and you aren't hurting anyone, and they try to take it away from you... burn them. (ﾉ◕ヮ◕)ﾉ It also helps to surround yourself with people with support you. A lot of us have become support for each other here on this website. Feel free to talk to me any time. ^^
Exercise and eat healthily. Exercise raises endorphins and makes one feel happier and healthier. It is certainly an easy and effective way to boost your self-confidence.
When you are feeling terribly insecure, write down your positive traits and read them back. This feedback would give you surprising results.
Look in the mirror and smile. Studies surrounding what's called the "facial feedback theory" suggest that the expressions on your face can actually encourage your brain to register certain emotions. So by looking in the mirror and smiling every day, you might feel happier with yourself and more confident in the long run.
I used to be incredibly insecure. I thought I was the ugliest person on the planet no matter what. Then I started modeling & ever since I started my confidence has gone up so much. I now know that I'm beautiful & amazing no matter what. It breaks my heart to know how much I used to hate myself because I really am gorgeous & wonderful. If you're interested try checking out Barbizon or Page Parks.