I would love the idea to develop games. How can I teach myself computer programming? What programs or software must I use? I have the new iMac?
first try rpgmaker XP or rpgmaker VX... (for rpg's)
or else try gamemaker... (its a bit harder but you can make any kind of game)
there are many forums that can help you...
good start and its simple...
then try harder engines like unreal engine 
thats how i started
try a book or google on some languages... its great help 
I know your end goal is to program games but I started off that way and got discouraged by the sheer complexity of trying to do it the old fashion way (actually coding it piece by piece) and the new way which is to use a program where you take big virtual pieces and plop them down in your configuration (nothing is new just new ways of doing the same thing in a different way, no coding at all, and always seems cheesy to me)
I would suggest that if you want to get started learning to think in programming terms go with figaro's advice start with something simple, and in my opinion you have a great opportunity with UNIX on your mac. One of the easiest to understand, good learning curve (to a point)
Start with console (Applications/Utilities/Console) and a list of simple commands, maybe a good unix introduction book like UNIX for dummies (Dont worry not many true lovers of "the code" out there will make fun of you for not knowing, were just glad to see you get started) and just start working with it. Most books will give you simple things like making and changing directories to get started which is to get you comfortable in the console environment. I would also suggest starting to use vim text editor (look it up) instead of text edit.
Once you have the basics of how to make a computer do what you want it to do, then other languages are not as intimidating just other ways of doing what you already know how to do.
Keep with it and dont get discouraged.
Learn from the pros, attend seminars, read their blogs.