You might start in theMac (or IOS) Developer Library
Specifically look at Programming with Objective-C for an introduction. You could look at iTunes U for some courses as well as iBooks.
If you're near a Community College you might want to look to see if there are any intro to programming courses offered.
One of the bigger hurdles will be getting comfortable with Xcode. It is really a great programming environment, Apple really nailed it with version 4.5 but it will take some getting use to. It's not like just opening an editor and entering code there will be a learning curve. Unfortunately there have been so many changes to Xcode over the years it is hard to find any good 3rd part documentation for it. Mastering Xcode 4 by Joshua Nozzi is not bad. It is for 4 but a few minor versions old so while it;s close there are differences. It might be worth a look.
One of the great strengths of a system like Xcode, an IDE (Integrated Development Environment) is that is is (in a way) self documenting. When you open Xcode if you look in the upper right hand side of the Xcode window you will see an icon labeled Organizer. Clicking that will open the help and documentation window
from there you can find help for Xcode as well as Objective-C.
Good luck