GMAT Preparation

First of all, both CAT and GMAT have different patterns. Although, both measure a person's abilities like Quantitative and verbal, but the content differs. GMAT is an adaptive test, which means as you answer the question right, a harder question will be given to you and if you answer wrong, a question easier or equal to your last question will be given. You can move forward only if you answer the current question. Whereas, CAT is a linear test where questions are numbered and you can go back and forth anytime and choose which question you want to answer first. The verbal ability parts of both the exams are somewhat different. While GMAT has Sentence Correction, Reading Comprehensions and Critical Reasoning. CAT has some similar question types but it focuses more on grammar and vocabulary. GMAT also has Analytical Writing Assessment and Integrated Reasoning which is different from Logical Reasoning and Data Interpretation questions in CAT. So Preparing for CAT may somewhat help, but since the pattern differs you have to study according to the question type and timing of the exam. Again, GMAT requires SMART studying whereas CAT requires a lot of practice.

I have seen people being 35 and still cracking GMAT to get into Stanford GSB. And you are just 18. Look for your current career prospectives. Complete your graduation with a good percentage. Build your resume am enough to stand out and to note building resumes means not just anything but things such sun help you to stand in a clear apart and would help highlighting the same in your profile. Then try to analyse why are you willing to give GMAT. The ifs and buts to follow. And for MBA in Harvard alike you need to have a minimum work experience of about 5yrs+. So you have got a lot of time in hand. Preparation hardly takes 6 months if you are dedicated enough. So time won't be a factor for you. Stay focused and stay blessed. Keep on questioning all your doubts and we will be happy to help.
Cheers mate and all the best.

Some reference books are not enough, if you're studying for MAT or GMAT especially on your own that is. Firstly, if you're studying on your own, make sure that you spend at least 6 hours a day with at least 4-6 months of determined attention towards your studies. Secondly, there are a lot of YouTube tutorials on the Internet that'll help you do the right kind of preparation and keeping the schedule. Finally, coaching for these exams separately or altogether will cost you a lot, so keep that in mind and fuel that as your passion. If you can study this way, you can make a difference. So don't just go by some reference book by the market. There are hundreds of books, plan your schedule properly and use technology along with your studies. Good luck and all the best for your future endeavours.

GMAT preparation books:
Veritas Prep Complete GMAT Course Set
The Complete GMAT Strategy Guide Set by Manhattan Prep
The Official Guide to the GMAT Review 2018
MAT and XAT preparation books -
How to Prepare for Quantitative Aptitude for the CAT by Arun Sharma (Published by McGraw Hill)
How to Prepare for Data Interpretation for CAT by Arun Sharma (Published by McGraw Hill)
Logical Reasoning and Data Interpretation for the CAT by Nishit K Sinha (Published by Pearson)
How to Prepare for Logical Reasoning for the CAT by Arun Sharma (Published by McGraw Hill)
Logical Reasoning and Data Interpretation for the CAT by Nishit K Sinha (Published by Pearson)
How to Prepare for Verbal Ability and Reading Comprehension for CAT-by Arun Sharma & Meenakshi Upadhyay.