Objective-C is a thin layer on top of C, and is a "strict superset" of C, meaning that it is possible to compile any C program with an Objective-C compiler, and to freely include C language code within an Objective-C class. Objective-C derives its object syntax from Smalltalk. All of the syntax for non-object-oriented operations (including primitive variables, pre-processing, expressions, function declarations, and function calls) is identical to that of C, while the syntax for object-oriented features is an implementation of Smalltalk-style messaging. - https://en.wikipedia.org/wiki/Objective-C
If you've downloaded the command-line client and have Objective-C installed on your machine, then go ahead and fetch the first problem.
exercism fetch objective-c
In order to be able to submit your solution, you'll need to configure the client with your Exercism API key.
exercism configure --key=YOUR_EXERCISM_KEY
When you've written a solution, submit it to the site. You'll have to configure the command-line client with your exercism API key before you can submit.
exercism submit PATH_TO_FILE