I want to use fog in blender cycles, but I don't understand the mist thing (I am a beginner). I have seen a tutorial on YouTube about volume scatter, but when I apply it, the render view and the rendered image are nothing but black. Does anyone know how to fix this?
When you give your environment a Volume Scatter shader, you need to use a light source other than a Sun Lamp, like a Point Lamp or a Plane with an Emission shader. A Sun Lamp essentially lights a scene from an infinite distance away. The light has to go through an infinite amount of space before it reaches your objects in your scene, and so a Volume density of anything greater then 0 will completely block the Sun's rays, since the Sun is an infinite distance away. So you have to use a light source that has a definite location, like a Point Lamp, Spotlight, or Emission shader.
If you must use a Sun Lamp as your light source, instead of giving your whole environment a Volume Scatter shader, create a Cube or Sphere and give that a Volume Scatter, and just scale it up so it encompasses the visible area of your scene. That way a Sun Lamp can penetrate through the fog, since it only has a finite distance the light has to go through.