What's the best resource to learn how to write apps for Mac OS X?
Aaron Hillegass's book "Cocoa Programming for Mac OS X" is certainly up there. It would also be well worth your time to learn the C language well.
To jump right in as quickly as possible, I'd recommend going through the following Cocoa Dev Central tutorials:
- http://cocoadevcentral.com/articles/000081.php (C)
- http://cocoadevcentral.com/d/learn_objectivec/ (Obj-C)
- http://cocoadevcentral.com/d/learn_cocoa/ (Cocoa part 1)
- http://cocoadevcentral.com/d/learn_cocoa_two/ (Cocoa part 2)
- http://cocoadevcentral.com/articles/000082.php (Style part 1)
- http://cocoadevcentral.com/articles/000083.php (Style part 2)
Once you've read those, you should be able to do quite a bit on your own, just using the documentation in Xcode (option-double-click on a class name in Xcode) and Apple's developer site.
You should start by learning Objective-C. Programming in Objective-C is an excellencent introduction to the language
If you're looking for a free resource this is an excellent web-site
Only after you have a solid understanding of the language would I worry about the different frameworks available.