Talk.
Every day, sit next to someone new and talk.
Someone that isn't associated with your current friends. It can be someone in your classroom, someone at lunch, or someone in a club that you've taken interest in. But you have to talk. No one will want to be your friend if you don't talk to them. No one will want to be your friend if all you tell are lies when you DO talk. No one will want to be your friend if you do not love yourself, because then they will think you are incapable of loving them in return.
So before you even begin to seek out new friends, learn to do that. Learn to love yourself. Look at your life, or search your very soul, for just one thing that is good. Maybe you have pretty eyes. Maybe you are good in school or have some kind of talent. Maybe, because you never talk, you are good at listening. Whatever your good trait or talent may be, use that to slowly boost your overall self-esteem. Expand upon it. Make a list if you want to. List only the good things, and if you write anything bad about yourself, immediately erase it (or scribble it out to the point where you can no longer read it).
Don't say "don't", "can't", "won't", etc. Only use "do", "can", "will", etc. You DO have potential, you CAN make friends, and you WILL be happy someday, if only you keep on trying and never, ever give up. =)