After 12th you can apply for National level Engineering exam IIT JEE or your state level exam, according your score you can get the college for B.tech Aerospace Engineering. For further to know the eligibility of IIT JEE you can go through with given below link.

JEE 2018

You have to appear for JEE and other engineering exam to apply for aerospace engineering.

You can check the details of engineering exam from given below link

Engineering UG exams

Below are the some of the top colleges for aerospace engineering

1- IIT Bombay

2- Hindustan University, Chennai

3- Indian Institute of Aeronautical Engineering, Dehradun

4-IIT Madras

5-IIST, Thiruvanatapuram

6-IIT Kanpur

7-IIT Kharagpur

8- Punjab Engineering College, Chandigarh

9-Park College of Engineering and Technology Coimbatore

10- University College Of Engineering, Rajasthan Technical University, Kota