Since Mac OS is built on Linux, you have a wide variety of command line programming options. If you don't know already, you can access the console via the "terminal" application in your application/utilities folder, or by clicking the magnifying glass icon on the top right of your screen, and then typing "Terminal".
As for programming language, I'd probably use Python. It's already installed on Macs and it has an interactive shell, which is great for testing out Python code quickly.
More over Python runs on Windows too. So whatever code you write on your mac, should work on Windows, too (just take careful note of pathing when moving from Unix to Window filesystems "\" versus "/" but you can use os.path. Read more
here).
Ever since learning Python, it's become my all purpose language since it runs both Mac and Windows (and Linux obviously). On top of that I do a lot of Microsoft Dot NET programming (C#, ASP.NET) and I can use Python now as Microsoft adopted Python as one of it's Dot NET languages via IronPython.
Couple notes: I learned Python first on Windows, which for me was easier to learn as the Python Windows installer came with IDLE -- simple GUI editor. It'll run each of your scripts in it's own shell, so it's a great tool for learning. If you go that route first, be sure to set-up your Environment Path to Python so you can execute Python scripts from any command prompt in windows (this is explain in the Python docs).
Also I don't know which version of Mac OS you have, but both Leopard and Snow Leopard come with Python. Snow Leopard comes with a newer version than Tiger, which is 2.6.
Read this.
There is a Python 3.0 for both Mac and Windows, but you may want to stick to the Python 2.6.x series for now as most user scripts are made with Python 2.6, plus that is what your mac has a default and if you ever go into Web Application building, the Django Framework, uses only Python 2.6. Plus then you can use Google App Engine later too if you wish.
