So I have a question, since the section of the 'how to run a game' isn't out yet I thought I would come to the boards and ask a couple questions;
1) Is there a book on Sigil, my players keep nudging twords that area.
2) This is obviously a high powered world, do I start them out at first level or elventh?
3) Is there any wisdom that you would like to pass on to new people? More specifically things I should do or avoid doing?
4) Is there really an up?
5) Is there a listing of 'spontanious plot points' that I can refrence from until I get my own thing going??
Thanks 
1) Check out chapter 7 of the PSCS for Sigil info. About 60+ pages of material on Sigil and Sigilian characters and plots.
2) ... if you're talking about Planescape, really you can play it at *any* level. High powered or not - it really depends on how you want to run it, and what you expect out of your players. Tell us more what you're aiming for and we can give you a recomendation.
3) Take your time with it and make sure your players are enjoying it as well. With players and DMs that are new to the setting it's often easier to run them as Primes new to the Planes, so that the players can experience the same learning curve as the characters. It's also useful to play up the different atmosphere of the setting (philosophy, intrigue, and mystery) to help them make the adjustment from classic sword and sorcery to planar mentality.
4) Only in planes with gravity. Usually accompanied by a down.
5) I'm not sure what you mean by this, are you looking for a series of modules to run players through?