CBSE 12th

For those serious about becoming a space scientist, here is what you should know and what you should do:
First thing to understand is, space Science deals not just with astronomy but also astrophysics, galactic Science, space colonisation etc.
Once the course is completed successfully, one can become astronaut, rocket scientist, meteorologist etc.
Opt for medical or non-medical in your class XI and XII.
Remember, you need to have Physics, Mathematics and Chemistry as your subjects.
You have to clear your XII board exams with excellent marks.
After XII, you have to clear one of the several exams like AIEEE, JEE etc to be eligible for admission into institutes/ colleges/ universities that conduct courses dealing with space Science.
One of the prominent institutes to join is Indian Institute of Space Science and Technology (IIST), Thiruvananthapuram.
It is a Deemed to be University under Section 3 of the UGC Act, 1956 and functions under the Department of Space, Government of India.
According to the institute's website, IIST is Asia's first Space Institute and the first in the world to offer the complete range of undergraduate, post graduate, doctoral programmes with specific focus to space Science, technology and applications.
The various courses conducted by the institute include B.Tech (Aerospace Engineering), B.Tech (Avionics) and B.Tech (Physical Sciences) at undergraduate level and various M.Tech and M.S. programmes at postgraduate level.
Among other institutes one can join to become a space scientist include Indian Institute of Science Education and Research (IISER), Bhopal, Kolkata, Mohali, Pune and Thiruvananthapuram; UM-DAE Centre for Excellence in Basic Sciences (CBS), Mumbai; National Institute of Science Education & Research at Bhubaneswar; Integrated Science Education & Research Centre (ISERC), Visva-Bharati, Santiniketan; PhD Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital; Indian Institute of Astrophysics (IIA), Bangalore; Indian Institute of Science, Bangalore (IISc), and various IITs.

Hi Prajakta,
You have not mentioned whether you want to know about careers for medical or non-medical stream. So, I will just give you a few options from both streams.
Non-Medical
Computer Application - If you are interested in computers and designing applications or software, CA is a great option. You can pursue either a B.Sc. in Computer Application or a BCA.
Aviation - You can choose to become a Pilot or an Aircraft Maintenance Engineer. To become a pilot, you will have to take an entrance to get through a flying college.
Architecture - If designing is what you are interested in, Architecture is a great field. If you manage to score well in NATA/JEE, you will be able to get through the finest architecture courses.
Merchant Navy - This is also a good career option for science students. You can get a B.Sc. in Nautical Sciences or join directly after 12th if you want to make this your career.
Defense Service - You can also join the Indian Army or Navy if you do not want to pursue engineering. It is great option and you get to serve your country too!
Allied Fields - You can also go into the research and development field, by pursuing a B.Sc. in Mathematics, Chemistry, Physics, Astronomy, Earth Sciences etc. If you are interested in research and academia, then this also a rewarding field.
Ethical Hacking - If you are the sort of person who is intrigued by this field, you can also become a certified ethical hacker. There is no preliminary qualification that you will need to become an EH, only knowledge of programming language and operating systems.
Engineering - Well this one is obvious. You can choose according to your interest, such as computer science, civil, mechanical, chemical etc.
Medical
Doctor - This is the first thing that must come to your mind. And it is a great field if you have the aptitude for it. This is not just another profession, but a big responsibility too.
Allied Medicine - There are other allied fields that will allow you to pursue your love of science, such as Radiology, Ayurveda, Clinical Research, Optometry etc.
Food and Agricultural Science -If you are interested in this field, you can pursue courses in Nutrition, Agronomy, Horticulture, Dairy Technology, Biotechnology, etc.
Life Sciences & Environment - You can also pursue courses in Life Sciences, Microbiology, Environmental Science, Biochemistry, Zoology, Genetics, Marine Biology, etc.
Make a good choice.
All the best.