The minimum I do is:
Map of the area
Map of the nearest bit of civilization
Name and Detail five important people (generally this will be (1) the local head of whatever; mayor, lord, etc (2) the best divine caster (3) the best arcane caster (4) the first potential plot hook and (5) a person that can join the party if needed.
Have a small pool of names to draw from for people they ask about (I need a better sword; I ask who the best smith is who'll deal with elves. | Ah, that would be Old Tolden, his smithy is that way)
A pantheon list, at least of the major gods, plus a note of major symbols, colors, a saying or two for the ones that most people in the party follow.
A calendar. At least one named holiday. I also decide some details about the moon(s).
I tend to run a lot of city-based stuff so...
By the second session I'll generally have a rough idea of the social structure of the city. I need to know who is is power, who hates those in power, what they plan to do about it, etc. I'll also know who the city lords report to.
I'll try to have three or four named people for the typical pool of people the adventurers will deal with early on: innkeepers, weaponsmiths, armorers, arcanists, alchemists, and entertainers. I'll try to have a couple notes about at least three inns or taverns.