When I create a world, it starts with a concept -- what is the campaign going to be based upon.
For example, my latest campaign is based on a professor's comment in a lecture I heard many years ago regarding the colonization of the New World, that one reason that the Aztec, Inca, and the like fell so quickly because the Europeans arrived during times of internal division, and that if the Europeans had shown up 20-30 years earlier or later the situation may have been quite a bit different. So I created a region that is much like the Central American coastline (more or less like Guatemala, etc.) with a not-quite-European area on the Eastern coast and not-quite-Mayan/Olmec/Aztecs in the interior. Then, just to make matters more interesting, I set up a not-quite-Chinese enclave on the Western coast.
After that I created the broad cultural outlines of each of the three groups, including bits on religion, politics, art, diplomacy, economics, and military. And, since the European-types would be technologically somewhat more advanced, I had to determine what set of rules I wanted to use for early firearms -- as it is they are slow, smokey, prone to breaking down, but cut through armour like nobody's business (which makes them primarily useful against heavily armoured European-types, but no extra use against the Mayan-types).
I also made an important decision at this point about adventuring in this world -- there are no sentient races that are not available to PCs (aside from a few undead, but that is a different matter) and there are only about seven sentient races. This means there are no orcs, goblins, etc., to attack. On the other hand, since there is a lot of jungle out there, there are a lot of wild animals (and monsters), including some truly vile plants, etc.
But where would the PCs want to adventure? Well, I now have a fully-developed city with many intrigues going on in it; equally, following the Mayan historical pattern, there are several abandoned cities now re-covered by the jungle, probably with fine art work (read: gold and jade/jadite), folding-books (potentially with magic), and guarded burial chambers (read: mini-dungeons). So between city intrigue, monster fights in the jungle, and ruins exploration, there seems to be quite a range.
I also started creating legends, vignette stories of daily life, superstitions, a calendar and holidays, and other little bits to flesh the world out.
Maps? Well, I've got a few of them, but given the maps that would have been available to the cultures at the time, "generalized" actually works. I a few better maps for my own purposes, but most of the geographical information that the players have is rather more sketchy (...suddenly I am thinking of Blackadder: "Do fill it in as you go along...").
So, there is a the good beginnings and, as I said, the world started from a concept and worked out from there.
