Dear aspirant....!!

For becoming an astronaut one should have

• best physical fitness
• good mental stability
• Bachelor's in sciences(all branches are counted)

There are two ways to reach(as far as i know) - Along with a bachelor's and master degree. One need to have a pilot licence from one of the Indian Defense Forces (highly recommended) or even commercial pilot licence would do.

Master degree,post doctoral degree  necessary with minimum 3 years work experience in respective field and make your way by satisfying respective space agency's requirements(NASA requires USA citizenship)

You should be clear about what you actually want to do by becoming an astronaut.

Since you will complete your 12th, you have a great way to go. Try being best(just give your 100% in whatever you do,believe in your decisions) wherever you are. Best academic records will ADD to your profile.

All the best!