I'd recommend a decent book, too: it's a complex language, and unless you get an end-to-end grounding in its peculiarities (and there are a number of them), you're almost certain to confuse yourself at some point. (There are definitely more opportunities to confuse yourself with Objective-C than C++, in my opinion...)
A book I like a lot is "Learning Objective-C 2.0", by Robert Clair, published by Addison-Wesley...