Hi,

You can appear for CAT in the final year of your graduation. Simultaneously, you can start preparing for the exam. To get into top IIMs, you would need to score a high percentile in the exam, at least 90 percentile and above. To prepare for the exam along with your studies, you should take out some time every day to practice. 

CAT has three sections - Verbal & Comprehension, Quant, and Data Interpretation and Logical Reasoning. To improve your first section, you can start reading newspapers, novels, blogs, etc. for Quant and DILR, start solving topic-wise mock tests available online free of cost. 

Here are some useful links for you to start preparing for the exam:

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All About CAT

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Best Books for CAT Preparation

Good luck.