It's not an easy answer. Since you didn't mention you specifically wanted to do Macintosh programming, I would probably start with Java. I will tell you why I recommend Java to start:
1. It runs on just about any box out there, and the code is portable across platforms.
2. It has a good balance of built-in safeness (applet sandbox, no pointers or pointer arithmetic, automatic garbage collection) and power to be able to do some serious things with it as you grow in proficiency.
3. It has a built-in, cross-platform GUI package (JFC/Swing, JSF) for building interfaces. Some great languages such as Python still do not have this, you have to add them on with extra bundles and learn the API (which is often written is some other language).
4. There is a huge amount of free information available on the web about Java, including many, many tutorials, help boards, etc. When you're stuck on a problem at 3am this can be a big advantage. There are also hundreds of good Java books available.
5. There are a number of good development environments available, scaling all the way from a simple text editor plus a command-line terminal, to full-blown integrated enviroments with all the goodies such as NetBeans, Eclipse, etc. Most of these are free/open-source.
6. Java is based on C and at some point you'll probably want to learn C and C-derived languages such as C++ or Objective-C. Once you know Java, learning C is pretty easy (that's the path I originally took).
