Well, a popular place to start for cities in Photoshop is Pyrandon's Eneini tutorial: http://www.cartographersguild.com/tu...photoshop.html

This style has the advantage of not needing serious architectural reference since it won't involve creating many (if any) individual buildings. In addition, it's well suited to making very large cities since it, again, doesn't depend on detailing each structure.

For guidance in Illustrator, and city design in general, I suggest searching the forums for posts by ravells.

And, of course, welcome to the guild!