There are a number of generations of Mac before OS X. What models and operating systems are you looking to write code for? Apple's MPW is free and supports a wide variety of hardware, but it's also got a pretty hefty learning curve. I'd suggest CodeWarrior for pretty much anything PowerPC, and Think C or Think Pascal for 68K work.
It *is* going to be quite bit different than anything you've done with OS X. I'd hit up Amazon or eBay for used programming books, and you'll also need to get your hands on the various volumes of Inside Macintosh - the specific ones will depend on how old you want to go and what you're looking to do.