The first step is to take Mathematics and Physics in 10+2. There are not many universities in India which offer an undergraduate programme in Astronomy, so you should go for B.Sc. (Hons.) in Physics. Then go for M.Sc. followed by Ph.D.; this will enable you to become an astronomer. If you are keen on joining ISRO, then one of the modes is through Indian Institute of Space Science and Technology (IIST), Trivandrum. IIST offers 4-year B.Tech. programmes and 5-year dual degree programmes and admission is on the basis of your score in the JEE Advance examination. After completion of the programme, students, on the basis of merit, are absorbed by ISRO.
Picture Credit : Google