One question often asked when deciding to build a world for a game or novel (but especially games) is: Where to start? Usually this question is asked or steered in the direction of creating a map. Therefore popular opinion falls into two camps:
Top-down: start by drawing a large scale map of the world or a continent and then add detail through smaller scale maps as needed.
Bottom-up: start by drawing a small area which is important to the beginning of the story/campaign and draw larger scale maps or other small area maps as needed.
Both approaches are often wrong.
You will get a better result if you start where you have the best ideas for the campaign/story! If you have several good ideas among different worldbuilding aspects (geography, cultures, pantheons, timelines/histories, local campaign area, etc.) then start with the one that is most essential or has the most impact to the others.
For example, if your story or planned campaign centers around a war of the gods, then maybe you should start by fleshing out your pantheon and let it drive your world’s timeline/history and geography and cultures. Perhaps the gods caused a cataclysm which impacts the world’s geography (maybe you’ll want to create before and after world maps.) In this scenario, since the gods are central to the story and are at war, what do they demand of their followers and how does that impact the world’s cultures? What world events have been caused over time by the war between the gods? How did it escalate? What were some other milestones? All of these can be part of the world’s timeline.