How can join ISRO after B.Tech in mechanical engineering?

To join ISRO, There is a written test followed by personality rounds to get placed. literally the notification on application for engineer's entry appears in the Employment news, mostly in the month of April(approximately) The centre for written examination is Bangalore. The date of Examination will be announced with the notification.

The questions will be mostly based on thermodynamics, fluid mechanics etc.

So do keep checking EMPLOYMENT NEWS and www.isro.gov.in

Hope this helps.