Tolkien started with elvish languages, Margaret Atwood with how her character eats breakfast. And you? You can start anywhere you'd like. In fact, you probably already have. But if you're officially lost on what next? or caught in the muddle of how the architecture of the place makes zero sense with the climate? Wondering what holes there are in the world you've created?
Worldbuilding is strange. Forget about one thing and suddenly your whole society is broken. So take the mnemonic EPIC LANDS (all credit to u/mr_nefarious_ on reddit!):
First, Environment. This is where you wish you'd paid more attention in geography class.
Then Infrastructure, Commerce, and Law & order: