Hi,
To become a aeronautical engineer after 12, the basics requirements are  to do btech or be in aeronautical engineering.
The basic eligibility criteria for a BE / B.Tech is 10+2 or equivalent examination, with Physics, Chemistry and Mathematics and must have a fairly high percentage of marks in the aggregate. One must also pass the qualifying exam JEE (Joint Entrance Exam) conducted by the IIT's.Selection to the graduate courses ( BE / B.Tech ) is based on merit i.e the marks secured in the final exams of 10+2 and through entrance exams.