Open the module editor (launch NWN and choose "Toolset"), start playing around with it, and read the document that Lightfoot linked to. And yes, be prepared to put a lot of time into it if you want it to be any good. Good but quick is next to impossible even for experienced builders.
Also, please tell us whether what you have in mind is a single player module, a multi-player module, or a persistent world. Do you want something you can play yourself, play on a network with other players, or host on a server like an MMORPG?