NDA

Harshal, you have many options like B.Tech, MBBS, BDS, B.Pharm, BPT, BVT, Hotel Management, BBA, BCA, BFA, B.Design, BJMC, Fashion Design, Interior design etc. There are many good private universities offering wide range of courses to choose from. Also many of the universities are offering scholarships and good placements.

Can anyone who has cleared his/her 12th apply for NDA, for Navy and Air Force, you need physics and mathematics as compulsory subjects.

Hi
To join Indian Army, navy or airforce as an engineer, go through the following links.
1. http://joinindianarmy.nic.in/
2. http://nausena-bharti.nic.in/forthcomingOfficer.php
3. http://www.indgovtjobs.in/2014/08/IAF-Career-Vacancy.html?m=1
Hope this suffice

Hi
Check your rank from rank predictor at shiksha and look for the cutoffs for last year to see where u can apply for engg. For NDA, you may see the details here for eligibility.
http://www.exam2016.in/upsc-nda-i-application-form-2016-17-upsc-gov-in/

NDA required 60%in your 12th and 6/6 vision and a tough exam so I will not suggest to repeat your class 12th instead you go for b.tech this year. If you wish to drop than you can repeat your 12th but it slightly effect your preparation for b.tech

yes you are eligible for jee main, and for jee advance you must have 75 % overall. you can improve your % by 75 while preparing for jee.
go for jee advance.all the best