Not to put [@BlackBishop]'s map work to waste, but there's also the option of choosing a pre-generated map via [url=https://bl.ocks.org/Azgaar/b845ce22ea68090d43a4ecfb914f51bd]Azgaar's Fantasy Map Generator[/url]. It's pretty customizable; you can choose how many regions the map has, add hills and islands, select a few different views (topographical, poltiical etc.), customize location labels and it can auto-generate borders. In short, it does a lot of the work for you and you can tweak to taste, giving you more time to focus on the writing.

Anyway; it's just a possible option to consider. As far as the cosmology goes, I like Bishop's ideas of having the current reigning gods be conquerors of an ancient pantheon, who then created the world separate from the ruined one they left behind. We don't have to explain the intricacies of how the universe works - only that which concerns us in the present, with our current pantheon. 
The conqueror route also gives our world a kind of classic 'bad guys' in the form of ancient races that followed the Old Gods, maybe some kind of demons that arose from their ashes, or what-have-you.