From the NaNoWriMo Tumblr Blog '20 Questions to Ask Yourself When Worldbuilding':

1. How long has your world existed?

2. How did your world originate? Do the people who inhabit your world have a creation myth or a scientific explanation for how it came to be?

3. What are some important historic events in your world? How did they contribute to the geographic or social structures that exist in your world's present day?

4. What is considered a curse word in your world? What is considered sacred, and what is considered profane?

5. What holidays does your world celebrate?

6. Does your world have a religion? Do the people in your world put their faith in some other sort of power or institution?

7. If your world has religion, is there one main religion or many religions? Are the main religions of your world monotheistic or pantheistic?

8. What resources are in your world? What are the imports/exports? Which resources are rare and valuable, and which are necessary or common?

9. What is the structure of your world's government?

10. What is the geography and climate of your world?

11. What are the seasons of your world?

12. What kind of calendar system does your world have? What's it based on (lunar cycles, a monarch's rule, etc.)?

13. What are the distances between important places in your world? Draw a map if you want to!

14. What is the class or social structure of your world? Where in that structure does your main character fall?

15. What language(s) do your characters speak? Is language ever a barrier to communication?

16. What are some details you can use from real-world places  that are similar to your world to make it feel more believable?

17. What does your world smell like?

18. What are some of the main dishes the people in your world eat?

19. What are the limitations of power, energy, or magic in your world?

20. What kinds of objects or ideas are familiar to the people of your world? What kind of objects or ideas are strange or outlandish to them?