I Want More Self-confidence

I'm a very quiet girl for the most part. I can let loose around close friends and that's it. I really want to know the secret to being so outgoing and being able to wear whatever the hell I want without a care in the world. I don't mean to be judgemental, but it's surprising how some girls wear clothes a little too tight for them or too small for them. Don't get me wrong, flaunting curves is not a bad thing but I've seen girls who usually wear an x-large shirt wear a medium.
Hell, I for the most part wear jeans and tshirts and if I wear a nice sleeveless shirt or tight dress, I put a coat on. I have tons of short dresses, but I don't leave the house without a jacket to put on or tie around my waist.