If you're on Tiger, great. Get Xcode 2.1 and go from there. Since you already know C and OO programming, learning Objective-C shouldn't be a problem. For a good book, there's Stepen Kochan's Programming in Objective-C, though it'll cover a lot of what you already know. Then there's Hillegass' Cocoa Programming for Mac OS X (2nd Ed.), which should be a good intro to the Cocoa API. That'll probably be your hardest problem. Cocoa is so massive, it may take you a while to be able to use all of it. Of course there's also Cocoa Dev Central and CocoaDev as online resources, and all of Apple's documentation.

Probably the hardest concept you'll run into are bindings, since they don't really have an equivalent in Windows. But they're awesome, and you should start using them as soon as possible.

Other than that, it's hard to know how to help you unless we know more about what you want to do.