If you are not familiar with the C Programming Language or the C++ Programming Language, you are encouraged to read the following book first:
The C Programming Language by Brian W. Kernighan and Dennis M. Ritchie.
The Objective-C Programming Language is a super set of the C Programming Language. Thus it is worth investing some time in reading the above book. There are lots of other books on the subject, but the book above was written by the creators of the C Programming Language.
Once you get a good background in C, you will find the Objective-C a pleasant language to learn and you will also have a smoother ride.