Clarify
Are you asking about how to use Xcode, or are you asking about how to write programs that have GUI elements (windows, text fields, buttons, etc.)?
If you're asking about how to write programs that have GUI elements, you'll need to move from C to Objective-C, and you'll need to use the Cocoa APIs.
Objective-C is needed as that's what Apple requires to access Cocoa. Cocoa is required as that's what Apple requires to access their GUI elements (as well as Apple's implementation of arrays, dictionaries, and other elements).
My advice is to find a tutorial on Objective-C and Cocoa. You will NOT be able to use C to create anything Mac-like on the screen. For instance, you will not be able to use C to write a line like "createWindow(x, y, width, height, title)". All of that is done in Objective-C by calling the Cocoa method that creates windows.