When you are practicing zazen, do not try to stop your thinking. Let it stop by itself. If something comes into your mind, let it come in, and let it go out. It will not stay long. When you try to stop your thinking, it means you are bothered by it. Do not be bothered by anything. It appears as if something comes from outside your mind, but actually it is only the waves of your mind, and if you are not bothered by the waves, gradually they will become calmer and calmer.