Remember that thread about mmo's you made?
Same answer, not quite as much work, but still a steep climb.
First of all you will need a server for your game to communicate with.
Second you would need to find a way to communicate with that server across domains.
Third, ok probably 1,2,3,4,5,6,7,8,9,10, etc, you need to learn php, javascript, some mysql, etc.