Science

Hi
You need 65% in 12th with either maths/accounts and English. If you are very sure of going for this course, you may start right after 12th. I would suggest you to go through the complete course structure before making any decision as that will give you more clarity about it.

Hi, refer to the list of options available here, use the predictor tool below
https://www.shiksha.com/mhcet-college-predictor
Hope it helps.

First of all congrats for completing 12th standard.
Any stream student can do BCA after 12th. In many universities it is compulsory to have maths but in some universities it is not compulsory to have maths in 12th standard. But in BCA there is maths subject.
All the best.

Hi
The best way is take pure sciences like physics to go for astronomy. IISER, IISC etc are best to pursue subjects like astrophysics. For space technology, mechanical, CSE, Electronics are the core branches If you plan to take b.tech + m.tech route. Institutes like IITs, nits also offer master's in it.

BCA,BSc IT / CS are courses with same structure and syllabuses. These are applied science / Tecnological courses. Job profile after these courses and higher education opportunities are also same.

BCA,BSc IT / CS are courses with same structure and syllabuses. Job profile after these courses and higher education opportunities are also same.

Hi
You can definitely go for MS. Just share your preference of country. If its US, here is what all you need.
http://www.msinus.com/content/eligibility-ms-usa-1435/

Idea is good. But, you have study hard.

A B.E or B.tech degree is always preferred than B.Sc +M.Sc in Indian industry until you plan to go for a research based career. Also, you may think of doing specialization in another field after b.tech as per ur experience so that will give you more options than integrated degree.