C or C++ would be a good place to start.
You need to first learn programming languarges before you learn API's. Start with C, as the majority of currently used languages stem from C. With OS X, you can do command line C programming--VERY handy.
Then, depending on which way you intend to head throughout the future, learn either C++ or Objective-C. Obj-C is what's used in Cocoa. Must everything else uses C or C++ (or Java, still stemming from C). If you intend to stick with the Mac for a long time, just learn Objective-C. If you intend to go multi-platform (good idea), learn C++ (so you know the industry standard), and then Objective-C (so you can do Cocoa).
Also, if you intend to program for OS 9, you would want to go Carbon and C++.
Bottom line: geta programming language or two down before you focus on APIs, like Cocoa or Carbon.