The Apple developer resources are excellent. Assuming you're a registered member, you should have access to the reference and sample code.
The key things to read up on:
- UI code. Much of it is quite similar (UITableView vs NSTableView etc), but you will have to make changes.
- UI guidelines. Mac users can be quite fussy about how good their apps look.
- Garbage collection. You don't have to use it (I don't), but it's no harm understanding it anyhow.
- Menu/button driven interface vs touch/gesture UI. Nothing too difficult, easier than iOS development, if anything.
Your model classes should be identical. Your controller code will need to be rewritten, but there are few if any extra 'skills' you need, as a Mac developer.