Srilakshmi, you haven't mentioned your highest qualification. Neverthless, to start your journey to become an astronomer, you have to study Science subjects preferably in Physics, Mathematics and Chemistry in your 12th. Now if you have completed your Diploma and it is after 12th, you have to take up Bachelor's degree in AstroPhysics or relative field. There are lateral entry options but not all Colleges offer this feasibility. After Bachelor's degree, you might have to atleast complete your Masters and Ph.D and apply for opportunities in ISRO, IUCAA, ARIES, NCRA, etc.