What the previous poster wclarkson means is that many of your Unix needs may be met within the OSX package if you dig. OSX is Unix based. So there is your direct route right there - no real need to get involved with Linux unless you want to.
You will have at your disposal a wonderful array of resources on the OSX DEVELOPER CD that comes with it, to keep you going in you Unix and programming investigations. Apple has done a great job here. OSX is worth the investment IMO, not to mention fun.
OSX has the Command Line Interface called "terminal" where you can develop your unix skills. Only beware, create dummy files to practise moving and file admin, cos it is easy to scru up and wipe them out.
As for a language, C++ can be appealing - but I'd recommend learning some 'C' first or simultaneously. OSX is built, i think, in Objective-C, which like C++ is an object-oriented superset of C. Java, ultimately, also has roots in C.
I'd recommend Metrowerks Codewarrior Discover Programming edition package, which includes resources for learning C, C++, and Java, and the Powerplant GUI. You may also want to check out a free Trial download of RealBasic. The network user group for this programming community is really vibrant and helpful.
I'm by no means an expert, but I just wanted to give you more info than simply "OSX".
Good luck