You can get selected in ISRO on different levels like technicians, assistants, juniors engineers, scientists etc.
For getting absorbed as an scientist you have two options:
1) After completing 12th from PCM subjects, appear in JEE exams, get good rank in JEE mains and perform well in Advance. Apply for IIST perform well in the courses, secure CGPA above 7.5 and you will
mostly get absorbed in ISRO. for more details follow the link:
http://www.iist.ac.in/admission/...2) Complete B.Tech/M.Tech/Ph.D. from any other college from a subject which is compatible with ISRO. Apply to ICRB exams, do good in written exams and interview and get absorbed in ISRO. for vacancy notifications in ISRO follow the link:
http://www.isro.org/scripts/job....