There are only three parameters to set to create fog in your scene easily.
Check the Fog On box.
Press the Fog Color button to pick a color for your fog.
Change the Start value of the Distance. This value defines the distance from the camera to where the fog starts. Everything within this distance is clear without fog effect.
Set the End value to decide the length from the camera that the fog ends.
Setting the Fog Color to black can simulate the spot light effect.