How make a mmo game?

Take a look at this question:

http://answers.unity3d.com/questions/3389/can-i-make-a-mmorpg-with-unity

Basically it's not easy most of the times to turn a single player game to a MMO but it highly depends on the game itself and the features that you want to add. I would say the easiest path is to use either unity's own networking (which can not have more than 64 players on each server) or unitypark suite because they both use unity on the server and you can share lots of code and make your game at least multiplayer if not MMO.

Different MMOs and multiplayer games require different features and there are lots of products available to help you. There is photon, unitypark suite, electrotank, smartfox, netdog, RoarEngine and many others but you should choose based on product features and your game.

Read this article to learn more about choosing multiplayer/networking middleware for your unity game