Hello,

Firstly complete your 10th with good marks, then choose Science (PCM) in 11th and 12th.
The basic eligibility criteria for becoming an astronaut is that a candidate should have Bachelors/Masters/PhD degree in the STEM field or Aeronautical/Aerospace engineering.
The best engineering college in Pune is College of Engineering Pune also called COEP.
You can get admission in COEP through MHT CET exam which you can take up after 12th.
I would suggest you to prepare for JEE mains and advance to and if you will be able to get into IITs then that would be great.

I hope this helps.