Why not learn just start with Objective-C on your Mac? Objective-C is a strict superset of C, so anything that's valid C is valid Objective-C. That said, Objective-C is for writing object-oriented programs, while C is for writing procedural programs. What that means is that while Objective-C contains all of C's capabilities and syntax, you don't actually use the strictly C stuff too much.
C++ is object oriented like Objective-C, but they're fairly different languages. Learning any object oriented language will make it so you understand the basic concepts/theory behind object oriented programming (OOP), but to be honest, if your goal is to learn Objective-C, learning C++ first probably isn't really the way to go.
I think Stephen Kochan's book is an excellent way to go. It teaches you Objective-C without assuming you've ever programmed before, so there's no need to learn C or any other language first.