Certification

Hi Reshu,
I recommend you the book of Stephen Kochan, is called "Programming in Objective-C" is aimed for those persons who has none-to-little background in programming, Stephen introduces the language very nicely.
Another good one is the book of Jeff LaMarche, "Beginning iPhone 4 Development: Exploring the SDK", is also very good.
And of course, the book of Aaron Hillegas, is called "Objective-C programming" is from the Big Nerd Ranch series, it's a classic and well-know book to learn Objective-C in an efficient way.
Other than these books, if you still feel unprepared, here are some other books that are highly rated for the preparation of Objective C:
1. iOS Programming: The Big Nerd Ranch GuideThis book is great for iOS start-ups. The authors of this book Joe Conway and Aaron Hillegass explains through the essential concepts, tools and various technologies involved in iOS app development.
2. Programming in Objective-CObjective C has become the standard programming language for application development for both the iOS and Mac OS x platforms.
3. Tapworthy: Designing Great iPhone AppsThis book is for designing the best exceptional user experience for iOS apps. This book doesn't teach you how to develop apps with Objective C.
Feel Free to raise more questions.
All the best.