How can i start towards this course?

For CAT preparations you need to start completely with basics in all the 3 sections quantitative, verbal and DI-LR. Having a strong hold on basics is very necessary because this CAT exam is all depends on your speed with accuracy. Without strong basics you won't be able to attain speed and accuracy.

For quantitative start attempting questions from books like Arun Sharma which is pretty good for quantitative ability. It explains basics very well and have all level of questions. You can increase your questions level once you are sure about your hold on basics.

For Verbal Ability you need to read lots of good books written by good writers like Charles Dickens and various good newspapers like Hindu which will help you in improving your English. For improving your vocabulary you should start memorizing 5 new words daily with their meaning and correct pronounciation and use them in your daily conversations so that you can memorize them properly. For RC section start reading passages and understand the thought process of author and what is his tone of communication. Also try to figure out whom that passage is targeting and what actually is the real message author wants to convey. This analysing technique will help you solve most of the questions asked below the passage.

Along with this start giving lots of online mock test and analyse each test's solution thoroughly which will help you in overcoming your mistakes and come up with a strategy to attempt your paper which will definitely help you in improving your marks and percentile. It will also help you in knowing your strength and weakness so that you can work on them.

All the best!!