GMAT

Exams like NMAT, MAT SNAP and CMAT are relatively easier than the likes of CAT IIFT and XAT. Though, among the easier exams mentioned, I feel NMAT and SNAP are worth giving due to the following reasons:-
NMAT - No negative marking; can book the exam as per your convenience; max. Of three attempts provided; no GK (if you dislike it); tentative cutoff for NMIMS 2017 = 209; clear the cutoff & a call is guaranteed; probably the costliest exam (1950 for one attempt); best college = NM Mumbai. SNAP Negative marking; no sectional time-limit and cutoffs (explicitly mentioned); GK included; best colleges = SIBM Pune & SCMHRD. When it comes to the other two exams CMAT & MAT, there aren't many top B-schools which accept their scores. You can keep them as a backup option, in case your unable to crack NMAT and/or SNAP. Hope it helps!
All exams have the same ranking you can use any of them. Good luck.

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.