You don't have to learn C first, but learning Objective-C will be learning C plus some. This gets debated fairly frequently here, with no real consensus. If you have use for C outside of Objective-C, learn it separately so you know what's what. If not, and your only goal is Mac/iOS development, probably dive into Objective-C. Steve Kochan's Programming Objective-C 2.0 will cover what you need of C, so that might be a good place to start "from scratch".
-Lee