I know you don't want to hear this but you have to feel beautiful from the inside first before you can feel beautiful on the outside. Trust me looks aren't the only thing that makes a person have a better self esteem. Even the most beautiful people have insecurites and even low self-esteems.
You have to become comfortable with your self and be confident abou the person you are. There is nothing more attractive than a person that is confident and secure with themselves. I guess to take steps to get to that stage you have to stop comparing yourself with the other girls in your school (perhaps the ones that are getting the most attention). Think of why you are different and what makes you special as a person, eg. you're smart, have a specific talent, beautiful eyes etc. Once you find the quailities you like about yourself, thrive on those because trust me EVERYONE has insecurites, even the most beautiful girl is school does. Once you stop worrying about how you compare to other girls you can start focusing on yourself then you can start to build your self esteem b/c no one wants to be average or like everyone else, find what makes you standout and don't say there isn't b/c I know there is, you just have to find it.