CBSE 12th

If you score between 96 to 99.5 percentile you have decent chances. Please check detailed cutoff below.
https://www.shiksha.com/mba/course/post-graduate-program-management-indian-institute-management-iim-a/1653

Admissions will be made on the basis of percentile/score in CAT-2016/GMAT, performance in academic writing test (AWT) and personal interview, academic (10th,12thand Graduation), co-curricular and extra-curricular achievements, and work experience. Start preparing by giving more mock exams.

Yes you can get into IIM Ahmedabad if you score well between 95-99 percentile.
https://www.shiksha.com/mba/course/post-graduate-program-management-indian-institute-management-iim-a/1653

Please check eligibility below.
Candidate must have completed graduation with at least 50% marks or equivalent CGPA (45% for SC/ST/PWD/DA) from a recognized University Final year student can also apply .Must have obtained valid percentile/score in CAT-2016/GMAT (at least 700) not before December 15, 2014(Overseas Indian candidates)
Note - It is mandatory for a final year student, who is provisionally admitted, to submit the mark sheet and degree certificate of the final year bachelors degree examination held on' or before June 30, 2017.

Hi,
You can think of these fields:
Physiotherapy
B.Pharma
Biotechnology
Nanotechnology
Microbiology
Astro physics
Wild life Science
Environment Science
Marine Science
Psychology
So, decide the field according to your interest and caliber.

St. Paul, Sri Satya Sai Vihar, Daly College, Emerald Heights, IPS are some of the good schools you can opt for maths.

You can sit for IIT-JEE, BITSAT, VITEEE, AMU, NDA entrance, IPU-CET and other state engineering entrance exams.

You are advised to appear for NDA exam conducted by UPSC, twice a year. Those, who clear the written test are called for 5 days Service Selection Board (SSB) to check for their suitability for commission as an Officer in defence services by rigorous evaluation of their Officer Like Qualities (OLQ).
It is followed by medical examination. Candidates those who opt for flying pilot branch in IAF will have to undergo pilot aptitude test. Candidates who are selected for IAF first undergo training at NDA - Pune followed by flying training at AFA - Hyderabad.

With 62 marks and 89% marks in JEE, your JEE rank would be low. I got 117 in JEE(115, being the cut-off), and 89.8%(ISC Board) and got CSE in JIIT-128.