Suicide should not be taken lightly, it is an irreversible decision that people who have survived attempts will usually tell you they regretted the moment after making the attempt. Even if you figure out a way to commit suicide that is painless it will not be pain free for the people who love you that get left behind to deal with the aftermath.
If you are contemplating suicide you should not be trying to find the pain free way to do so but instead finding a way to talk to family members, friends, doctors, teachers, or others that might be able to give you some help and advice. There is always a way to turn your life around. In the end you might find that the true pain free way was to find a new lease on life.