Astronomy Entrance Exam

Dear Archita,
Certain points in this regards:
(#1) Only PCB wouldn't help. Some entrance exam requires maths as a mandatory subject also because of the huge involvement of physics, mechanics and astronomy. It is really important to have strong understanding of physics, chemistry and biology. Do not worry if you are not good with maths as you can still do something with the other subjects. You must remember that as far as Astrobiology is concerned you will be mainly self-taught.
(#2) Astrobiology, astrochemistry are some field which are under-evolve in India. You know that there is only one serious, dedicated institute in India which is sepcilized in this field.
Indian Astrobiology Research Center - They offers International Diploma programme in Astrobiology. However to widening the scope, you must to go abroad.
(#3) After your Bachelor's, Masters, and essentially Ph.D. you will work as Faculty or Research Analyst in Earth Science department or IARC itself. Nothing steps involved in it. IARC offers 4 months of Diploma in astrobiology.