Dear Piyush, for becoming an AstroChemist, you need a solid understanding of Chemistry, Astrophysics and other relevant fields. An AstroChemist is a part Astronomer and a Part Chemist who works extensively in the Space Context of the Periodic Table. For such a profilw, you need to have studied Chemistry as one of your subjects since your 10th. So starting with your 10th standard, keeping in mind the subjects that compliment your dream, you should ideally go for Mathematical stream though it isn't mandatoy. In your 10+2, you should study subjects like Physics, Chemistry and Mathematics. In your Bachelor's degree you should have studied one among Physics, Chemistry and Biology as your subjects. Then you can pursue your M.Sc in AstroChemistry as your specialisation.