No, Objective C is neither C, nor C++.
Objective C was an attempt to graft the Smalltalk concept of "everything is an object" into a C syntax. It is not very C-like, nor is it particularly Java-like.
C++ is an attempt to add classes to C without doing too much damage to the C syntax and compilation. The result is not very pleasing, and it doesn't do anything more than C, just much less readably.
Java's syntax is based on C, but it has classes built in at a more fundamental level than C++. Many strengths of C (in particular, pointers) were stripped out to make programming in Java "safer". However, it's not a very good OO language either. One particularly painful omission is the lack of a destructor. Because of this, objects cannot deinitialize themselves as they are destroyed; the creator must explicitly deinitialize objects. Still, Java is a pretty good language, and the classes that are provided give a wealth of capabilities that are available on all the platforms supported. The chief downside of Java is slow performance, as it is interpreted bytecode (a la UCSD Pascal).
Objective C is compiled, but because everything is an object, performance is nowhere near as good as C or C++. I wouldn't recommend learning any other language as a stepping stone to Objective C, as it differs significantly from all others, except perhaps Smalltalk. While Objective C compilers exist for Unix/Linux, the "frameworks" (class libraries) used in Cocoa are unique to Mac OS X.
Squeak is an implementation of Smalltalk for the Mac. It may be worth trying.
IMHO, of these languages, only Java is suitable for a beginning programmer (one with less than five years experience). The others are just too complex.