Hi Ajay,

You can apply for CAT in the final year of graduation. To prepare for the exam, you should start preparing for it at least 6-8 months in advance. However, it actually depends on your level of knowledge of the topics covered in the CAT exam which are Verbal Ability & Reading Comprehension, Data Interpretation & Logical Reasoning, and Quantitative Aptitude. You can refer to Arun Sharma's books to prepare for the exam. Also, join a coaching institute to get timely guidance if you feel. 

Good luck.