In your case, either C or C++ are good languages to learn to begin programming on a Mac. Both OS X and the iPhone OS use Objective-C, which is a superset of C, so while you will still have to learn a new language after you're done with the class, you'll have a very good grounding to build from, and there is lots of overlap. Both C++ and Objective-C are built from C, so you'll be in great shape, I think.
Unless you have lots of free time, though, I'd suggest holding off trying to learn Objective-C and Cocoa until you're done with your class, otherwise you may just end up confusing yourself.
Oh, and find out beforehand whether the class uses Windows APIs or if it's a general C++ class just using the Standard Library. You should probably ask the teacher, as your counselor probably won't know. If the class uses Windows API, then you'll have to put a copy of Windows on your Mac. If you are just using STL and such (which I imagine should be the case at your level, unless you end up doing GUI stuff) then using OS X's XCode will be just fine.