Assuming you are a 12th student so this is not possible to get into the NASA/ISRO after passing class 12th as for getting into NASA/ISRO you have huge knowledge about Aeronautics and Aerodynamics. So you need bachelors, masters as well as phd degrees in this field. Also you must have many years experience in BARC (Bhabha atomic research center) or ISRO(Indian space research org). However, you can take admission in Aeronautical Engineering or space engineering after passing class 12th as this would be the first step for the same.

Hope this helps!!!