Depends on when you want to appear in the exam. If you want to take the exams next year, then you should start with a good textbook right away, and study from the Thirty years war to Peace of Westphalia, and then till the Second World War, study all the major historical events and their significance to international relations. After that, you'll have a general idea of how nations interact with each other and then you should study all the theories plus the principles governing the Foreign Policy design, and other topics like warfare and terrorism. Take it one topic at a time and you'll be able to do it no time.

If you're going to appear in 2019,then k highly recommend Henry Kissinger's book Diplomacy, or his other book World Order. Both are excellent introductory texts for International Relations, and while they're not textbooks, they're still very good books from one of the biggest diplomats on the planet right now. If you study them before studying the subject of IR, everything will become a whole lot easier.

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