If you want to make native iOS apps, then there really is only one sensable choice, and that
is the Objective-C language.
Yes you could learn C++ or Java to make iPhone apps, but they would not be able to
harness the power of the Cocoa Touch Frameworks, in the same native way that
Objective-C can.

If you want to make Mac OSX apps, then you could choose any of the C languages, or java, or any other number of programming and scripting languages, but again I would personaly
recommend learning either Objective-C, or C++.

If you decide to go the Objective-C route, then these books listed below, I would highly
recommend, they are written by people who teach Objective-C, and iPhone programming,
in highly acclaimed coarses.

First read this one, it will teach you the basics of ANSI C, and lead you on to a good
understanding of Objective-C.

Amazon.com: Objective-C Programming: The Big Nerd Ranch Guide (Big Nerd Ranch Guides) (9780321706287): Aaron Hillegass: Books

Only after reading and understanding the work in this book, should you move on to one
of these two books, which are also great learning tools, but dont move on to one of these,
unless you really get the first one.

Amazon.com: Cocoa Programming for Mac OS X (4th Edition) (9780321774088): Aaron Hillegass, Adam Preble: Books

Amazon.com: iOS Programming: The Big Nerd Ranch Guide (3rd Edition) (Big Nerd Ranch Guides) (9780321821522): Joe Conway, Aaron Hillegass: Books

Hope this is of some help.

Regards Mark