I am in 12,as an average student how can i be an aerospaceengineer in isro/space research centers studying phisics,chemistry,maths,computer

I have completed 10(CBSE)scoring 96%,.As only an average student how can i be an aerospace engineer in isro/any other fields?Now i am doing 12in state board.i would like to go for iit .how will i get admission there .i have wrote aptitude test for kvpy ,but didn't get.can i write it this year.