Getting admission in IIST is the easiest ticket to get into ISRO as a scientist/Engineer. ISRO often picks up graduates and post graduates from IIT's and other reputed college so its better to choose a course like Aerospace, Mechanical, radio or engineering physics which have more chance to be shortlisted. 

ISRO also conducts ICRB for recruitment for graduates. Individuals are tested as per their branch, after clearing the interview one can get appointed as a scientist or engineer. 

Hello Anamika

If you are in 12th and in science stream you can persue BE/Btech and join ISRO as Scientist/Engineer.

If you do BSc, you can join as scientific assistant.

If you do diploma in engineering, you can join as technical assistant.

If you do BCom, BCA, BA any other graduation, you can join as assistant in admin.

If you do MBBS, you can join as medical officer.