I am doing a research project on adolescent and I believe that a good first step for you would be to tell people how you are feeling, talk to your old friends about it, your parents, a teacher or guidance counselor.
do you have anything in your life that you would say helps you feel better?
How long have you been feeling this way?
The first move to increasing social life, would be your confidence. Gain it first. Your next step would be, meeting these people and talking about recent events in your life. Ask about their lives as well. Comment on events they mention. A strait forward meet up isn't always the best, so call them, video chat, or IM. Those options work as well.