There are many resources available for Objective-C. Here is the guide written by Apple for using the language (Objective-C) and the framework (Cocoa).
Programming with Objective-C (Mac Developer Library)
Here are some things that are important that you should know.
- Variable Lifetimes (where and when variables are accessible)
- Variable Typing – Strong AND Weak (variables containing objects)
- Reserved Words (words that can’t be used to name variables/methods/classes)
- Naming Conventions (how to name variables/methods/classes)
Pages for setup and installation are also provided: