I tend to start with feeling first, mechanics to follow.
Partially this depends on how you are planning to present your material. Is this for people already in your game? In that case I would suggest discrete articles -- places of interest in City X, types of government found, great events in history, regional variations in culture, etc.
If you are presenting it to people who are not already part of the game, I would suggest starting with a mood piece. "Out of the jungles came the sons of Akapa..."
Overall, however, here are the kinds of things my players expect when I put out info on my homebrew:
* a short piece (1-2 pp) on Why We Are Adventuring Here. Often this is in the form of a myth, a short tale of a local event, or something like that, but essentially it sets a theme and a mood for the whole campaign.
* a list of any changes to character generation (certain classes in, out, and/or altered; restrictions on magic, if any; allowable races, etc.). This is always important.
* a recent history with names and at least vague dates. This lets them know what has been happening; this also usually relates back to the first point, so the players feel part of a continuum.
* at least one general piece on local culture - styles of dress, type of government, naming practices, gods worshipped, etc. Often this breaks into multiple pieces on these various topics.
As an example, here are the files I have up at my game group's website:
* Tales of New Mavarga (an introduction, 6 pages, to local history and culture, introducing the races and the type of game that is going to be played, heavily swashbuckling with low magic in a jungle/New World setting)
* Approved Character Classes
* Approved Prestige Classes
* Approved Races
* Approved Skills
* Approved Feats
* Revised Weapons & Armour List
* Languages of New Mavarga
* Recent Timeline (goes back about 75 years)
* Character Background Options
* Character Questionnaire
* Classes and their relations to Occupations (everyone has a Day Job in this game)
* Major House Rules (primarily regarding magic and combat)
* Basic Prices in New Mavarga
* Theudurb & its Environs (the local major city; includes notes on government and guilds)
* Holidays in New Mavarga (religious, secular, and borrowed)
* Jadism and its Variations (the major religion of New Mavarga)
* Mavargan Name Elements (a guide to creating character names)
* Concerning the Klon Hall (the secret society the characters are attached to)
* Important NPCs (a list of known Good Guys and Bad Guys)
* Other Nations and Regions
* Concerning Akapan Society (small notes on a nearby race, little know to the PCs)
* New Mavargan Military (how the local armed forces are organized)
* Sir Ufbert Comes to Theudurb (an introductory tale to get people in the mood)
* Akapan Creation Tale (a story all of them would have heard, even if they don't understand it at first)
**whew** I wrote more than I realized...
Anyway, that is one person's campaign
