The very first step to the World Building approach is to set your scope. Scope concerns the size of your world and can be as small as a phone booth or span multiple universes. I even remember a silly game I played as a kid that consisted of two lily pads, two frogs, and some flies flying around on screen. Man, my brothers and I played that game for hours! I use Lord of the Rings and Star Wars a lot as my examples because they are classic stories that provide great examples for many of aspects of story telling (and because I'm a huge geek!). The scope of Tolkien's Lord of the Ring's trilogy was western Middle Earth. Lucas's scope in Star Wars spanned a galaxy, of course.
Once you set the scope you have defined boundaries, set limitations for yourself in much the same way that the first level of an outline does. From there your world and your story will begin to take shape before you ever type your first word.
Here then are some questions to ask yourself when thinking of the scope of your world.
1. How big? Multiple universes, a galaxy, a solar system, planet, continent, island, or a couple of lily pads?
2. What is(are) the shape of the world(s)? Normal sphere, flat, broken, Dyson sphere, other?
3. What is(are) the moon(s) like if any? Are there any rings around your world?
4. Atmosphere? Nitrogen/Oxygen, Methane, Tibanna, or even none?
5. What is the climate like? Think Hoth, Tatooine, Kashyyyk, or New Zealand.
6. What is the terrain like? Flat, hilly, mountainous, caverns, rippled by some huge cosmic event?
7. What are the "water"-ways like? Are the oceans of water, liquid nitrogen, or some other fluid? Are they shallow or deep? How do the rivers, lakes, and inland seas define your world?
8. What are major flora and fauna considerations for your world? This is not about defining specific kinds of organisms, but broad definitions. Maybe there are only microbes, or insects and a few small grasses. Perhaps there are massive mushrooms or huge predators or even deadly gas-filled behemoths floating among the methane clouds.
9. Is the core of your world molten, ice, or an immortal sleeping dragon?
These are of course pretty big concepts. If your scope is smaller, take these down to a smaller scale but ask the same questions. And certainly you aren't limited to these. Hopefully the questions above will get your creative juices flowing.
And just to give you an example, the book I'm working to get published right now (Islands of Loar: Sundered) is a book about a planet that exploded and the people now live on a few large chunks floating in space. Only magic keeps them alive.
Obviously the world your characters live in and where the story takes place can have a great deal to do with why the story happens the way it does. It shapes culture, politics, economics, nations, and sometimes even races.
Next time we'll take a look at world building from the cultural level.