Science

Please refer www.ssbcrack.com for getting knowledge of defense related post.

You can apply for IISER after class 12th. The ways to apt for IISER is given in below links:
https://www.iiseradmission.in/?page_id=8.

Since you have not got admission, you must try again. You can go for M.Sc Physics or M.Sc Astronomical physics. You can study MS or post-graduation from abroad by giving some exams like GRE or IELTS. These are entrance exams for admission in post-graduation.

Yes, you can appear for NDA exam in class 11th and the first post rank given is Lieutenant in officers.

For joining Indian Army, you have many ways. After class 12th, you can apply for NDA TES entries. After graduation, you can apply for CDSE, NCC, UES, TGC, ACC, SCO, PCSl. In all above case, you have to clear SSB interview to get selected. Also, eligibility for all entries is different. Check it with

On official website, admissions might be closed, because you are too late now, but do try from management and offline forms.
All the best for your bright future.

For joining Indian Navy you can apply for various entries such as:
NDA (after class 12)
UES
10+2 B.Tech entry (through JEE Mains score)
NCC Special Entry
SSC entry

You can but you should have maths as well.

1)A.I.I.M.S
2)A.F.M.C
3)J.I.P.M.E.R
4)Manipal university
5)S.R.M MEDICAL COLLEGE
All the best for your bright future!

LIT-Nagpur, MIT-Pune, Dhol Patil-Pune are the colleges for your consideration.