Look at yourself in the mirror and find features that you like about yourself. Features that you're glad to have and really like them.
Write out all your positive traits or traits that you consider to be your strengths as well as your achievements. There might be a time when you've done something you thought you never could do.
Write out a list of your flaws or what you consider to be your flaws. Reconsider them and think whether they're actually valid/true and if they are, learn to accept them.
Exercise - not only will you look better but you'll feel better about yourself (For me it's an easy way to forget about my problems and only focus on what I'm doing at the time) and you'll give off a better energy.
Find some small things that you were always afraid of doing and try tackling them one by 1. Preferably starting with the smallest.
My confidence increases when I have the best work of anyone in my class. When I work really hard and achieve the results I want, my confidence rises. That wan I can look back at my accomplishes with pride and say, "wow I can't believe I did that."
Well, think about it. How did all the confident people around you get confident? Were they somehow born with the confidence gene? Don't be ridiculous. When you were a baby, were you really self-conscious? Don't think so. At some point in your life, you read magazines, listened to criticism from people, etc and you formed certain beliefs that are holding you back.
You need positive beliefs. People mistakenly believe that one I get a certain haircut, a certain paycheck, a certain car, THEN I'll be confident. That's wishful thinking and patent nonsense. A loser with money is still a loser.
First do the things you're scared of. Then you get the courage. People think incorrectly. They think you need confidence first to do things. You just need to go out and do it like how actors just do it on stage.