XCode is a multi language development environment. You can do Java, C++, C and Objective-C (and more, I'm sure). You need to decide what you are going to learn, usually by deciding what you're hoping to accomplish. If you want nice, natively integrating OS X applications, go with Objective-C. If you want _simple_ portable code, go for Java. If you want slightly more difficult portable code, go for C++ and C.
Get a book on one of these languages and you will be fine. In my experience, books are all about the same. The main thing is perseverance from you, the programmer.