ответить на письмо 120-140 слов it's not that i don't to be sociable i'm just terribly shy and don't have any confidence around others.i'm afraid that others students in my class will laugh at me if i say something wrong.so, everyone thinks i'm a snob and that i don't talk to them because i don't like them. i don't know how to change their minds about me . have you ever felt like this? please help!

My dear friend! Many people many times felt just the same as you do right now, I'm not an exception. This is a widespread problem of bad self confidence. And this problem is solvable! If your friends think you're a snob, then go ahead, make them think differently. Walk into the classroom and smile to your classmates, say hello to everyone. It's scary only for the first time. And don't worry about their reaction, they're just surprised. If you're still too shy to do that at once, then do it another way: go to an after school activity. That can be sports, music, drama, anything you choose will fit. There you'll get to know different people, it will be easier make the first impression, just because they don't know you and they won't judge you by the things you did before, like the classmates can. 
So, here I finish my letter. I hope, those advices can help you.
Always yours *имя*