First off, for all intents and purposes (and I hope I'm not offending anyone here), you, the author, are God. On this world, you are the creator, the Alpha, the Omega, the end-all-be-all. The head honcho. The big cheese. The...You get the point. So lets say in all of your Godly wisdom, you want your world to have a sky color that's something other than blue. Green maybe. Fine and dandy. You're God. You can do that. But hold on just a moment. Now, as a writer, you have to go and find out exactly what it would take for a world to have a green sky. Our sky here on Earth-Prime is blue because of our atmosphere and the mixture of gasses in said atmosphere. So what gasses would combine to make a green sky? Do some research and Methane will pop up. So now you have a planet that has a methane-rich atmosphere. Problem solved, right? Not so fast.
Now, along with your green sky, you've just jumped right into what types of creatures would inhabit the planet. They don't breathe oxygen, now do they? Nope. They're methane breathers. So what kind of creature would evolve into such a thing? And for that matter, what would an oxygen-rich environment do to them? And what would they exhale? And where would that methane come from? On earth, the plants breathe in the carbon dioxide that we exhale exhale oxygen. So what do these creatures breathe out, and what filters it back to methane? Lots of questions there, and the answers are part of what you (God) has to come up with.
If your characters are interacting with humans, you've just stepped into a whole lot more of detail. What size is the planet? What difference does that make, you ask? Glad you did. Smaller planets spin faster and therefore have shorter day/night periods, which the inhabitants of that world would take as normal. Your human characters could be driven buggy by thirty minute days and nights. Also, smaller planets have higher gravity because they spin faster and are more compact. So big strong jock from earth might just get out-muscled by a toddler on planet quick-spin, if he can even lift his own bulk.
A few things that you, as God, need to consider for your world:
- What stage of development is it? Genesis? Pre industrial? Highly industrial?
- Planet composition - Is it iron core? Is it carbon based? What's the atmosphere like?
- Dominant species - Creationist or Evolutionist, you still have to figure out how the dominant species on the planet became so.
- What rules of chemistry, physics, or other odd bits of science do you need to know in order for your species to survive, and for your planet to function?
This is just a very (VERY) brief overview. There is much more to be covered, and much more that you, little Godling, must uncover before your world will breathe and live on its own. And there's so much more to this: Societies in your world, energy, laws, rules, religions...All of them things that make your world function, and all things that you must consider when you're creating the setting of your fantasy, sci-fi, horror, or other novel. If you'd like to know more, one of the best books on the market on world-building is, in fact, titled World Building. Who knew, right?
Okay...That's enough for now. Back to your characters! Write on!