Since you're learning on a Mac and want to be able to work up to Objective-C. I would suggest finding a good book that you can understand on C. It doesn't need to be a Mac-specific book. You may still need a few pointers on using the Mac tools, but if the C book uses Unix for its examples, then you should be able to figure it out.
Once you get some understanding of the basics like variables types, functions, loops and pointers, you can start learning Objective-C if that's where you want to go. You don't need to be a C guru, but Objective-C is built on C.
You may also be interested in Java. Java has a similar syntax to C and would get you right into objected-oriented thinking from the start. OS X has very good Java support from what I can tell, and Xcode 1.5 has Java code-completion now.
Once you get the basics of any language though, you can apply those skills to picking up new languages fast... so don't feel like you'd be stuck with whatever you choose to learn first. Learn what looks interesting and like something you'd stick with for a while.
