Hello Suanta Kundu, If you want to pursue Chemistry after 12 then the best thing that you can do integrated 5 year course(B.SC + M.SC) in
chemistry from IIT like IIT Kharagpur provide, or you can study 4 year B.S course from IIT like IIT Kanpur provide.if you don't get a seat in IIT then you can chose NIT that is also good.
Other than IIT's and NIT's you can also Pursue B.SC in chemistry from other reputable college like
Hindu College, Delhi NCR
Archarya Narendra Dev College (ANDC), New Delhi
Ethiraj College for Women, Chennai, Tamil Nadu

But the best will be studying from IIT's.