Cocoa: where to start?

Recommended resources for beginning Cocoa with Objective-C
NSOrderedDescending

do the temperature converter in the Cocoa Application Tutorial

short articles at Cocoa Dev (here much more here; C review)

Aaron Hillegass's book

Cocoa Fundamentals Guide (here)

Specific to PyObjC:

Will Larson's tutorials (here here here here here)

Apple's page including a version of the temperature converter

Read the official introduction carefully (my biggest problem)

A page of links to old material with simple demos of specific Cocoa features that mostly still work here.

Code a simple game like TicTacToe, Fifteen, or Color Sudoku.

After that, I've got tons of projects here and here. Get started with bindings (here), then move on to Vlad the Impaler (here).

Learn specific topics by reading the Apple docs (slowly and repeatedly, it can take a while to get it, by building a simple demo project that does only that one thing. Like NSPredicate which we've done in six posts (here here here here here & one to come).

And if you want the PyObjC templates for Xcode see here.