networking advice, please

I've created an RTS game, but I don't know how to make the game multiplayer - how to be able to play it over networks or the internet.

Now I need to change the code to make it multiplayer-playable-online. I really want to do this the easiest way possible. Where do I begin? where do I go to learn what I need to change to make it networkable?

Thank you for any advice you give me.

You will have to have started coding for network play. It is possible to implement it after making a game but it involves going into nearly every script and configuring it for network play and adding network view components. Why don't you check out Unity's networking documentation