Admins and moderators are the overseers. They can either lurk through the topics, keeping up with the game, or react to others' reports. They distribute the rewards, too. Well, it's in their name: they run the place.

To start, you need a forum. Any forum will do, but the more modifiable it is, the better. The person who registers the forum is the first admin, which often means that they can't be relieved of their duty; rules vary from hosting to hosting, and you can write your own rules if you host it, by selecting the engine that serves you best or writing your own.

In Russia, many forums put in a lot of effort to get the best design for their story, which means that the stylesheets ought to be modifiable as well. It doesn't have to be this way, but it helps with immersion. Also, oftentimes there are features that you'd like to put in that aren't in the vanilla engine: medals/achievements, gifts, reputation system (where people vote up or down on certain posts of a player, thus creating a number that represents their respect within the community) etc.

From my experience, the world is expected to be included when the game starts. You can register a forum and fill it up with info, or you can write it beforehand and open the forum already filled with details. It's not impossible to create it as you go, but it runs with a risk of turning the world into a kitchen sink or an outright mess if done poorly (i.e., without moderation and/or by newbie worldbuilders). Most often, two or three persons cooperate to fill the world under a certain single idea - "your typical fantasy", "post-apocalyptic world filled with advanced tech and magic that's dangerous for the mage as well as for the surrounding world", "My Little Pony-based" etc. - with material moderated by the authors themselves through discussions. Usually, players are forbidden from writing the world because they're assumed to be not in the groove with the idea, which may or may not be true, given how people can still play inside the world with joy and cunning.