You'll need to add the CoreData framework to your target, create a data model, and instantiate the NSManagedObjectModel, NSPersistentStoreCoordinator, and NSManagedObjectContext objects.

Adding Core Data to an existing application is discussed briefly in this Apple document (search for "existing application")

You should also view the Apple tutorial to get a feel for what is involved.

You can always consider just using SQLite as well.