Obj-C is very, very, very easy to learn if you already know C, as is C++ (though not as much as Obj-C, in my opinion). I'd suggest that you start by learning C, and then if you still want to learn Obj-C and Apple's APIs you look into GNUStep.
Also, if you want to program for the iPhone note that you'll probably want a Mac, since that's the only officially supported option.