There's different levels of activism. Still, I go on river clean ups and volunteer on Halloween at a local church. That doesn't move mountains. You can go the route of good 'ole community service and go plant a tree. That's great, wonderful, and I hope you do.
Community service can be lots of fun and really satisfying.
But then again, you could organize an international Green Day, or something else with a better name, where people all over the world go out and plant trees. Even better, you could make a statement along with that with media coverage and all, explaining why the environment is so important. I got involved in all this stuff after a very strange seventh grade. I saw there's a lot wrong in the world that can and should be fixed. There's no reason I or any other young person can't start now to try to solve some of those problems.