"I think, therefore I am."
If you think you're beautiful, you will be.
If you just can't shake the feeling that you're not (which is untrue), then look to the world and say, "Up YOURS. I am BEAUTIFUL." Keep telling yourself that and it will come true.
Write your own story. Stop comparing your looks with other people's.
Start doing things for your own benefit. Get a new haircut--something edgy that you wouldn't have tried before. Try a new style of makeup--you'd be surprised at how much little tricks enhance the beauty that's inside. Don't however, under any circumstances, start to think you NEED makeup to be pretty.
Go to the gym. Feel the blood pumping through your veins, feel the flush on your cheeks, and marvel at the power of your own body. The human body is the perfect unity of form and function. Put yours to the test physically. When you succeed, you'll have a better appreciation for it than you've ever had.
Drink more water. It'll improve your complexion and just make you feel better.
Change your eating habits. There are wonderful foods to be had that do amazing things for your health.
Challenge your mind with new subjects. They don't even have to be school related. Become informed on a subject most people aren't, like a particular president, a rare disease, African American literature (no, you don't have to be African American), the mating habits of bees, whatever. The more unique, the better. There's a certain satisfaction in knowing that you know something someone else doesn't, and it'll increase your self-esteem.
Be spontaneous.
Basically, what all these suggestions are getting at is this: Set your own expectations of personal beauty. When you learn to focus on them, the expectations of others will fall away and lose their power. And THAT, my friend, is what makes people take notice.
The sun is shining. Step into it, and you'll forget what it was even like to live in the shadows.