First of all, take Science as your stream in senior secondary. Then, aim for a very good score in the competitive exams like JEE to get into a reputed institution, like IISC or the IITs which offer aerospace engineering. Complete the four year course of aerospace with a really good score. Try on for ISRO.

Hi ajinkya

To become a cosmologist firstly complete your class 10 then +2 with science stream (PCM) and then get admission in universities which offer B.Tech, M.Tech, MS, PhD programs in Avionics & Aerospace engineering . To become a cosmologist, take admission to the undergraduate programmes offered by IIST. For that you have to appear for the IIST Admission Test (ISAT) conducted every year .Indian Institute of Space Science and Technology offers undergraduate course. There is an eligibility criteria for appearing for the test to seek admission in IIST. You have to get minimum 70% marks in +2 examinations with science as your subject. relaxation of 10% is their for reserved candidates.

Good luck