Well, I can't speak for anyone else, but I usually start with a name.
Names are important. Knowing someone's name was once thought to give you power over them. In many ways, names continue to define us.
So,if I was going to start a new campaign, I'd find a name to anchor it too.
For this example, let's use the name Thornhold.
When I think of the name, Thornhold, it immediately conjures up something out of a dark fairytale. The ruins of a once-great castle, surrounded by a forest of thorn trees. Haunted thorn trees.
Immediately, elements of story are starting to present themselves: a fairy tale setting where 'happily ever after' never happened or happened so long ago it's no longer relevant. And a spooky, haunted forest. Except, it's not haunted by ghosts. Or just by ghosts. It's haunted by the Fey.
We could totally build a whole new setting just out of the description above; a D&D campaign set in a dark fairy tale world. It would work. That would totally work.
But what about Thornhold? It's the name of a city, obviously. An isolated city with a mysterious past? Maybe. It's isolated by the Thornwood. Why is it isolated? How was it founded? What was it's purpose? Is it still fulfilling that purpose or has it gone in a totally different direction?
First things first. Let's figure out how old Thornhold is. Roll a die. Get a 2.
So, Thornhold is about two hundred years old. Is it a proper city? Or a large town? Maybe a city with the population of a large town? So there would be lots of empty buildings and abandoned, decaying houses? Gothic and spooky.
But let's not get distracted. Thornhold is 200 years old. It's isolated. There are no other cities or towns near it and the Thornwood is difficult to get through. Why build a community there? Maybe it started as a military outpost? Maybe it was a place-holder for an expanding empire which wanted to harvest the resources of the region. That would work.
Only there were difficulties. Economic troubles. Political unrest back home. In the East. Exploitation wasn't feasible but an isolated community would be an ideal place to exile people you couldn't risk martyring. Political exiles. Social agitators. People you'd want out of the way but not necessarily dead.
Yes, that sounds good. Thornhold became a city of exiles and rejects. But how do they get there? Overland? Too slow. Too dangerous. There could be a river passing through the city. It would make sense to build a community near a source of water. So, there's a river. What to call it? Names have power. Do we want to give it a fairy tale name? No. Let's go further abroad. Instead of fairies lets look at angels.
A quick Google search gives me a good name. Cassiel. The River Cassiel. The Cassiel River. It flows through Thornhold and the Thornwood. Bisecting both. Rolling east through the woods and maybe some mountains? Mountains would be good. Just to reenforce Thornhold's isolation. Yes, we'll have mountains.
The Trollhammer Mountains. The Trollhammers. Heh. Back to the fairy tale stuff.
So, people get to Thornhold via the river. Just the river? What about other ways? Magic? No. Thornhold is isolated. There's got to be a reason teleportation spells don't work. Maybe it's in a low magic area? Maybe spells above, say, level three don't work in the city? Is it natural? The effects of a curse? An enchantment? Is it like that everywhere? Or does magic work differently in different parts of the world? That might be interesting. We'll make that cannon.
Magic in the world is in flux. Maybe because science is on the rise? We'll decide later. But high level spells don't work in Thornhold.
Back to the city. Exiles arrive by boat. Just boat? Maybe airships. Gaily colored blimbs? Rigid zeppelins? Tech here is maybe more 18th century than Medieval? That could be interesting.
Ooh! There would be firearms! Sold!
So, back to air ships. Why not literal air-ships? Ships made of wood. Magic wood. Wood that's lighter than air. Light as a feather. Featherwood. Air-ships made of featherwood, expanding the Empire's reach! I like! So, people arrive in Thornhold via the River Cassiel and the occasional air-ship.
But what sort of city do they arrive in? It's got walls. With a name like Thornhold, it's going to have walls. Serious walls. Thick and tall. Gothic walls. Gray stone walls enveloping a city, bisected by a river. Bisected north and south.
How many neighborhoods does Thornhold have? Time to open PhotoShop! Open a Hex Map file, fill in some hexes to represent the walls. Okay. Got an outline of the city. Now we add the river and then we start breaking it down into neighborhoods/districts.
Seven. Seven districts and seven is a magic number. It's a lucky number. So, seven districts. What to name them? More fairy tale stuff? Eh. Maybe something more rustic. Things start to pop into place.
Wolf District in the north, then Sabre, Pine and Falcon. The River Cassiel splitting the city. The south districts: Faith, Greyhart and Key. Named for the city's seven founding families.
Hmm. If the city is separated why not the Thornwood. Maybe there's two forests. The Thornwood to the south of the River Cassiel and something worse to the north. Something darker.
Oooh! Fey are sometimes divided into opposing courts! I'll put the Seelie Court in the Thornwood and the sinister Unseelie Court in the Darkwood! And Thornhold in the middle between these two warring factions!
Okay, so what we've got so far: Thornhold is an isolated city, established 200 years ago and used as a dumping ground for Imperial troublemakers. The city has high stone walls and seven districts. High level magics don't work there and, just outside of the city walls, are the Thornwood and the Darkwood.
What next? Probably more history. ^_^