Hello there,
Imagine that you haven't read all the planescape books yet but is willing to become a planescape DM and start a campaign anyway. None of your players have ever played planescape or even heard about it. Your plan is to read the basic stuff in the campaign setting and start a campaign of, say, 30 adventures, and while advancing the campaign, read more and more about the subject, and present the world to the players slowly, with the help of images, for example.
My question is: how to start your campaign? just for starters: would you guys start it at Sigil or at some place in the outer planes?
sorry if I'm too crude...
Personally, I think you have to consider things in terms of game play.
For example, you need somewhere as a "home base". Most Planescape material assumes this is Sigil but I imagine you could use anywhere from a good or neutral plane (e.g. a town on the Outlands, Arcadia, Bytopia, etc.) This would be relatively safe but could have just enough visual oddity to drive home to the PCs that they aren't just in another generic medieval kingdom.
After they get their bearings a little bit (personally, I recommend that the PCs never get to feel that they know everything about the secrets of the planes) then you can inject allies and antagonists. The factions often serve this role (and provide an early opponent that isn't as lethal as a horde of demons) but races unique to the planes can also fit this role while injecting a little "oddity" into the campaign
Past that initial set up, if you want to run a long-term campaign; I supposed you should come up with a few larger theme threads to which you can string a number of adventures. Since the planes are about ideas, these could be themes related to larger themes like good vs. evil, law vs. chaos, creativity (see Infinite Staircase module for an example of this). Or you could tie it to some unique feature of the planes (see the Great Modron March for an example of adventures tied around the theme of the modrons traveling from plane to plane)
If you are just looking for recommendations for specific products, let us know as there are a number of modules that are popular among most of the PS community
If you are looking for unoffical ideas for adventures, there are a number of threads on this site. I recently had one just for adventures set in or around Arcadia
/forum/single-plane-mini-campaign-arcadia