ICSE is the doorway to your future
ICSE is your first public exam and therein rests its significance. While you may have appeared in scores of exams in school, ICSE is going to be different because you will be competing with thousands of students and the questions will be set not by your familiar teachers but by the ICSE board. Even the evaluation will be done by people who know nothing about you. So it is your exam paper that is all that can impress the examiner to give you good scores.
ICSE is neither the first nor the last exam of your life. But yes, it is the most important one. Because this is the exam which will set a benchmark that will be used to evaluate a lot of things about you.
All this is not to scare you away but to impress upon you the unique importance of this exam in your life. On an immediate level, ICSE scores determine your admission to the plus two levels. But if you reflect a little, you will realize that scores are but numbers. What is important that you should be thorough with the basic level of knowledge that the ICSE syllabus is designed to impart to you. This knowledge has to be gained not just at the mnemonic (memory) level, but at the comprehensive one. You must be able to comprehend the subjects and reach a level of understanding that is required of you.
It is only when you are able to internalize this knowledge and its comprehension that the learning will be truly fruitful and that you will be led to an enriching world of knowledge.