Mad Dawg has a nice reference here: http://forums.taleworlds.com/index.php/topic,290414.0.html
Although it may not answer all of your questions it's a good start. I would click on a few of those clans and see the format.
For specific advice.. you need a few people (to start I would recommend 3-4..) on board with you. You can always recruit more later. You need a clan purpose (Competitive? Siege or fun?) and then you can start the process. Make a clan post on here, get a voice chat server (Recommend TS3 or Mumble), and then some stuff that is optional. Website, forums, clan server on Warband, etc. It's not much different than making a clan on other games.
If you want to be competitive that's another story. You will need a solid foundation (leadership, skilled players) and 8-12+ total members. Hold weekly or monthly training, play on POM or other battle servers as much as possible with your clan. If you are planning to go this route I would recommend baby steps. You won't be top NA in a day, week, or even month.
Either way you're going to need to find some like minded people and get it going.