When you are world building the cosmology can influence everything. I usually start with the tropes/typical choices for the world/setting/genre that you are building. This makes things simpler for you and your players to relate to the world.
The first part of the Cosmology creation is to get the physical layout of the setting established. This typically includes a map of at least the starting point of the campaign and at a minimum an understanding of the world around that. I recommend at least enough so that players are able to relate and understand the setting. A world that is one city will prompt different characters than a world where the majority of it is a desert wasteland.
I usually also begin to create the meta physical planes and layout if it is relevant to the setting. This may include a pantheon of gods, any variations in natural law, whether or not psionics exist and any "unexplained" differences the players will begin knowing about.
The easiest way to think about this is to think of it as the rules the world follows. If it is not in the rules of the game, or different from the rules as written it is safest to make sure the players are aware of it. This will also lend an internal consistency in your storytelling and aid in the suspension of disbelief of the players.
