What should be my approach after class 12? I have an interest in space science, especially astronomy and astrophysics.

There are multiple couses available which you can follow after 12th if you have interest in space science, astronomy and astrophysics. Bachelor of Science is a great course for this because it has many specializaions, inclining to your interest like:

• B.Sc Physics with Astrophysics 
• B.Sc  Space and Ast