To start brushing up on your C/C++ skills, I would recommend practicing some of the techniques taught at
http://www.cprogramming.com/tutorial.html. Or check out the "Starting C" thread in the C/C++ forum. Once you're fairly confident with the more advanced techniques, you should go right into OpenGL. You'll probably want to use GLUT for your windowing, so that not only is your code portable, it's also much simpler. You can find more information on starting OpenGL on a Mac in the sticky at the top of this forum.
You also don't need to worry about learning Cocoa/Carbon for Mac OS X, because GLUT kind of abstracts you from that lower-level stuff, allowing you to concentrate on the important part: OpenGL.
> Is XCode alright for making games in?
You're darn right Xcode is good for making games. It's what the professionals use to create Mac games nowadays (and pretty well any kind of Mac program), so it's probably a good idea to use it.
In case you haven't already noticed, I have a Mac, and although I'm no game programming expert, I'll probably be able to help you out if you get stuck. So don't hesitate to post here.
Hope this helps