I mostly have advice, but alot of the advice I got from a series of Dragon Magazine articles run about a year ago (articles are still going, but not as relevent now). I wish I could remember the name ofthe article...
Anyway, here goes.
Start by coming up with broad ideas of the type of campaign you want to run, going so far as a theme if possible. Keep those ideas in your head for everything else you do. (examples: Ravenloft-horror; Dark Sun-environment; Dragonlance-romantic fantasy/good vs. evil)
You should probably come up with the type of region (climate and terrain wise) you want the players to start in. Once you figure that out, try to think about what kinds of regions would logically fit into place around it and get together a sketch-map of the area.
Then try to pinpoint it down to a base of operations for the players. A single city, or possibly even a particular keep or building. Planescape had Sigil, Dragonlance seemed to migrate towards Solace, Ravenloft had Barovia, Dark Sun had Tyr, etc.
Detail the goings-on of that area meticulously. You want to know who tells the stories, how the people react to threat, who they go to, guilds, leaders, NPCs, underground activities, economy, trade, common problems, what types of food they eat, music they listen to, what the language or accent sounds like - absolutely everything, as if it were a real place. If you're looking for how this can be done, go to a FLGS and flip through a Ravenloft Gazeteer (I've heard the Greyhawk one is good, too, but cannot vouch for it).
Once you've done that, try to establish the ways the area communicates with surrounding places. Use the economy you detailed from before and try to think of places where they can get their imports and send their exports. Maybe detail some major traders in the region or merchant's guilds. Get a decent idea of what's around the area. Try to include at least one serious threat, but not something that low-level (or whatever level you want to start out with) PCs cannot do something about. You don't want them to be able to vanquish the problem in a single adventure, either, though. They should stay in the region for a couple of levels while you detail farther and farther out, letting the players inch into your new creations.
Think about each of the major races (and monsters) in the campaign. Where do they live? What is their culture like? How do they interact with each other? Get a good idea of this so you can logically place them in adventures. For uniqueness, I like to take at least one monster and make it more civilized and common for player use (Dragonlance has Centaurs and Minotaurs fitting with its romantic bases, Dark Sun had Thri-Kreen, Ravenloft has Calibans [your traditional slightly off mutant, like hunch-backs], Kingdoms of Kalamr has hobgoblins, Forgotten Realms has... everything, etc.). This is a good way to let the players know right from the beginning that the campaign is different than others. Unless there is a race concept that you can come up with that directly supports your theme, don't create new player races at first. They tend to distract from rather than add to the campaign.
You should now think about the divine entities in your campaign (as it's important for the next step). Just get a good feel of who the major gods are. Try to come up with at least one god that is best for each class or race. A single god can be the best choice for multiple classes or races though, if you want a small pantheon, or you can come up with a different god for class groups in each race (a Dwarven spellcaster god, an Elven fighter god, etc.). Don't worry about creating separate pantheons for different regions at the moment (if ever; Forgotten Realms is the only campaign I can think of that does this, most just change the name and aspects of the established gods).
Now move on to the classes. Are there any classes with special restrictions or major overhauls? You'll mostly come up to this with the major spellcasters, but it's possible that there would be changes to the more militant classes. It's hard to think how Fighters could change or wouldn't fit a campaign, but Paladins might be restricted to worshipping particular gods. Same might be done with Druids and Monks. You may want to remove spellcasting from some classes (Rangers are probably top choice in my experience, but Bards could be done without traditional spells, but instead with more bardic music abilities). Unless you are a rules person, don't go overboard. None of the classes NEED to change. But if a tweak here and there would go a long way towards promoting the themes of the campaign, go for it. Clerics almost HAVE to be looked at, though, due to the domain choices and the different gods.
Now work on organizations. Only the most far-reaching ones need be worries about now. Wizards might all belong to a single order (a la Dragonlance), Paladins might have to go to a specific training and meditation camp. Most importantly, try to think of some of the major villains who might appear later. If you want an evil cult of scorpion worshippers later, you might want to know ahead of time to drop hints or foreshadow. Some of these might spark ideas for prestige classes, but think long and hard about whether it's necessary to have a prestige class to represent the order. And even if you decide to have one, it doesn't have to go to level 10. Alot of times a 3 level prestige class works even better (especially a prestige class appropriate for players who adventure and are away from their organizations for long periods of time).
Next you should look at every area you have detailed so far. Determine the major players. Don't worry about statting them out, race/gender/level is sufficient for almost any NPC. For any important NPCs try to come up with at least one personality quirk and one secret. For instance, the country leaders of the players' base of operations might be a benevolent family of werewolves. This accounts for the lack of a locally minted silver coin and an historic propensity for state-endorsed moon holidays. Figure out who knows about the secret as well. These are you adventure hooks for later on. Or the players may never know, but it has given you good background for role-playing the NPCs.
Try to create a few dungeons (if you like dungeon-delving, some people don't). It's a little jarring when the players find out there's a huge, sprawling dungeon a day's walk out of town that has never been there before. Try to come up with a plausible reason they are there. Abandoned dwarven ruins, the catacombs below a now-demolished castle, the burrow system of some huge unknown creature that is long dead, etc. They don't need to be near the base of operations, but there should not be any other major places on the way there (don't put another city on the way to the dwarven ruins if the players are focused on their base city). This can be difficult to logically do, but the game is Dungeons and Dragons so some leeway is alright.
From this kernel of a campaign, you can extrapolate as needed to create a whole world full of places to go, new people to meet, and opportunities to kill them.
