Specific to PyObjC:
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.