I think many of us find that there's a bit of a "void" in our lives where the alcohol used to be. I know I did!
I filled the void with three things that I'd always loved: exercise, reading and cooking.
Maybe one of those would be fun for you, too (and exercise is GREAT for your mood). If not, then maybe join a club at your school, or volunteer for the upcoming election (whichever side you're on, they are sure to need help).
Whatever you do, pick SOMETHING and go do it. Nothing succeeds like success.