The most important aspect of any book next to character is setting, for without it there would be no context for the story to exist. I would go so far as to say anywhere from one third to half of any story has to do with establishing the surroundings in which the action can unfold, depending on how familiar the setting is to the intended audience. The process of world building is going to be different for each author, but here I’ll offer what advice I can from my own experience.
Laying the literal groundwork for a story can be one of the most daunting tasks in creative writing of any kind, but also one of the most fun. For it encompasses not only the physical environment around the characters, but the kinds of people, creatures, and cultures that inhabit it as well. For nonfiction works, or historical fiction based on actual localities in the world, this task can be significantly easier as long as you do your research. In these cases, it’s really just a matter of describing people and places in a manner that readers who are less knowledgeable about the subject can picture in their heads.
In other kinds of fiction, however, you have to build up from scratch. Of course, it helps if you have a certain existing place or culture in mind when you create your own–most authors do–but the more wild you get, the more difficult it’s going to be to simply draw on existing material. And of course, in genres like science fiction and fantasy particularly, you want to give a sense of being as new and adventurous as possible. Remember, the readers of speculative fiction are generally looking for something to divert them from the world they live in; not plunge them deeper into it (though that’s not to say it should be impossible to relate to–especially in the case of allegory).
With physical settings themselves, it’s probably easiest to be as descriptive as possible. Naturally, it’s preferable to weave these descriptions into the action of the tale, but sometimes there’s nothing for it save to spend a paragraph (or several) simply giving details about how a place looks and sounds. If there are any kinds of distances involved in your world, it is also advisable to include a map somewhere in your book (or books, in case of a series) to help readers get an instant picture of the physical and political geography as you envision it. Having them done before or during the writing process can also help you remain consistent when giving descriptors of distance and direction in your actual narration, as I can attest to.
The same rules essentially apply when describing the societies that inhabit your world–especially if there are more than one, and if any of them are other than human. However, it’s usually possible to incorporate a little more action in establishing the essential characteristics of these societies. Living (or at least animated) creatures are typically more prone to moving about than mountains, and thus lend themselves more to showing rather than telling. And of course, in the case of communication systems, there’s no better way to establish language or dialect than through straightforward dialogue.
As always, it pays to be consistent when creating something new. If you have a world where the mountains are located to the west of a town, don’t have someone in that town refer to them several chapters later as lying to the east (unless you’ve got some legitimate sci-fi explanation, of course). Likewise, don’t turn a journey of three days into a journey of ten next time unless there is some reason it takes longer the second time.
These are just basic outlines regarding world-building, but they are essential to a story’s believability. Like I said, it really depends on what kind of story you’re writing as to how much effort is involved, but suffice it to say, if you find your readers–or even yourself–deeply engrossed in a work you created, you’ve probably done a good job.