Sr,iam vikram. I`m intrested in space science. Now i`m studying 12. I wish to join in ISRO or NASA. What should I do next?

Well there are many paths you can enter a space agency as
two of the primary are
1) Base Engineer/Researcher
2) Space engineer

the 2) category will propably require you to work atleast 5-10 years as 1).
The 1) itself has many many subranched right from astrophysics to mechanical engineer to robotics, and for all of them you need an engineering degree. I believe college is the best time to sort out your interests, So I will just underline your steps assuming your goal remains the same

1) Get into a good college (yes it matters), in your preferred branch
2) Maintain good academic record at college and more importantly build your profile in a certain specialization (like robot design/ gravitational physics ...)
3) Get an MS/PhD in that profile, yeah they don’t hire Btechs.
4) After even PhD you need atleast 2 years of work experience
5) Then apply and hope for the best

It’s a long and tedious journey, but if you are really interested then go for it, never say never :)
All the best