|Well First you will need a Mac with X-Code installed. It's free to download on the Mac App Store.
Then I suggest to follow some Tutorials on the Web to create simple Applications before you consult the Apple Documentation - where you can wuickly find solutions for specific problems.
In my opinion it's useless to buy books, as the Cocoa Framework is updated quite frequently and the books get outdated pretty soon.
What helps are Tutorials on the Web (do some Google research) and Apple WWDC Videos.