You have to consider geography, climate, cultural groups, history, language, and culture (gender roles, fashion, taboos, values).
These questions below help as starting points to help design a world, based on reality or fantasy.
- What’s the temperature like? Is it a verdant area or an arid wasteland?
- What’s the wildlife like? Any new species of fauna or flora?
- What’s their choice of cuisine? Is it vegetable based or meat oriented? Do they salt their foods for preservation or use spices for flavor?
- What language(s) do the people use?
- What beliefs do the people hold? What mottos do they chant, what political ideologies do they espouse? Are they religious? If so, are they monotheistic or polytheistic?
- What system of currency does society use? Do they barter? Is it all electronic? Is the unit of money universal?
- Would their fashion sense shock people of modern sensibilities? Are they scantily clad or modest? Do they weave their own threads or forge their own armor?
- What are their gender roles? Is it a matriarchal society or a patriarchal one?
- How does their society interact with foreign cultures? Are they xenophobic? Are they at war? Do they import or trade?