Set your foreground colour to black and your background colour to white
Create a new layer.Get the lasso tool and draw a rough cloud shape about half the size of the layer.
Go to Filter - Render - Clouds.
Hit Control and T...this will select the layer.
Get hold of the handles that appear and stretch the cloudy image...it will atart to look loike fog.
If it is too much in the layers pallete reduce the opacity.
If you need more mist...duplicate the layer and move the mist to where you want it or again hit control and T and stretch it agian.
Get a soft edge brush and set the opacity to about 10 or 15.
Brush over the mist with white to create some highlights or black to make bits darker.