As an author, you do hold the world in the palm of your hands when creating your novel. This is particularly true if you are writing science fiction, fantasy or paranormals (the SFP from now on to save some time).
What's the first thing to do on your way to building a world for your novel? The first thing to do is to decide what kind of world it is, namely:
Normal World: In a normal world, the SFP elements exist beneath the radar of most of the normal world's inhabitants. The rules of the normal world are still followed. A good example of an SFP in a normal world are THE CALLING Vampire novels.
Alternate World: In an alternate world, the SFP elements are known to the inhabitants and the world is very similar to a normal world except for the changes imposed by the existence of the SFP elements. Urban fantasies are a good example of alternate worlds. In particular I would recommend Kim Harrison and Kelley Armstrong for wonderful alternate worldbuilding. The JD Robb books are also a good example of an alternate world, but one set in the future this time.
New World: In a new world, all the rules of a normal world are gone. You will develop all the rules, lands, people, culture, foods, etc. Many fantasy books are set in new worlds and they require quite a lot of worldbuilding. An excellent example of a new world can be seen in Jacqueline Carey's KUSHIEL series.
Once you've decided on what kind of world you are going to build, what do you do next?
The next thing you want to do is establish the rules for your world. In a normal world, that would involve the rules dealing with the SFP elements, whether the inhabitants know about them, etc. It will also involve creating the rules for the SFP underworld that exists in your normal world. The same goes for an alternate world. While the inhabitants in an alternate world are aware of the SFP elements and those elements play a role in their lives, you will still need to craft rules for your SFP world to follow. Finally, in a new world, you will need to craft EVERYTHING and with sufficient detail so that the reader understands the rules of this world and how they impact on the story you are trying to tell.
How do you create these rules?
One of the best ways I've discovered is for you to think of your world's inhabitants (and / or your readers) as visitors to a foreign land. You will need to provide them with enough information and detail that they can understand the rules of this foreign land. Some of the things which visitors to a new world may need help understanding are:
Language: Will your characters use ordinary language or do they have special words related to the SFP elements ala the Harry Potter incantations? In a new world, you may even have the characters speaking a totally new language that you've created. Make it simple for your readers to understand the context of this new language and its words. The last thing you want is to toss out so many new words / languages that the overall meaning of your novel gets lost or bogged down with the worldbuilding.
Food: What do your inhabitants eat? How do the foods / dietary needs relating to the SFP elements get satisfied. In a new world, you will have the option of creating totally different foods and or creatures to be eaten. Again, strike a balance between this worldbuilding and the story line.
Culture: What is the basic culture of your world? Is it founded on Judeo-Christian beliefs? Arabic or Asian influences? Is it current / pop / urban or futuristic or historical. Consider the elements that most would understand given the culture of the times and provide explanation for those things which are different from that cultural norm.
Roles of Men and Women: What are the respective roles of men and women in this world and in relation to the SFP elements? Are women equals? Do they rule like the Amazons? What are the acceptable sexual roles of men and women and even more importantly, what is culturally acceptable regarding sex in your world. Is it puritanical or anything goes?
Politics: What is the political structure in your world? Democracy, theocracy, dictatorship? How does one attain political power and keep it? Does the political world impact on the SFP elements? For example, in alternate worlds, there are often political acts which have granted the SFP inhabitants certain rights or which have enacted rules governing how the SFP elements may not act.
Weather and Geography: What does the physical world you are building look like? What is the weather there? It may sound insignificant, but weather and geography can sometimes be key elements. For example, think of Frank Herbert's DUNE novels. Almost all of the political, cultural, food, business aspects arose as a result of the climate of the planet.
If you get hung up on whether something is working or not, step away and ask yourself, "If I was a visitor to this world I've created, would I understand this? Would it make sense based on what I know of this land ? "
If the answer is yes, you've done a pretty good job of worldbuilding. If not, you may have to go back and rethink the information you've got in your novel. But always remember, it's the story that will keep the reader reading so make sure that you don't lose the wonder of that while you are busily creating your world.