fog of war help?????

how would i go about creating a fog of war type thing, i am using a plane for the enviroment i want to create a blacked out fog that you can get rid of by exsploring the enviroment like in command and conquer, dune 2000, and other rts games, want it to work in the exact same way

game is 2d top down - galactic civilisations type of thing

help is gratly appreacheated.

forgot to say the programming language i am useing is c#

this doesnt realy help it gives you the script i want to learn how it works nd how to do it is there any tutorials out there??