After 12th or during 12th you can appear for IIST exam ( Indian Institute of Space Science and Technology (IIST), Trivandrum ). (ISRO) conducts the entrance exam for admission of eligible candidates in its Bachelor of Technology courses under different disciplines. IIST graduates above a certain pass percentage cut off in final B Tech exam are directly absorbed in ISRO system as Scientist/Engineer -SC grade.One can also join ISRO after finishing B Tech from university system. For this they have to appear for ISRO centralised recruitment. The advertisement for this is often published in national newspapers.You can also join ISRO after completing M.Sc, M Tech and Ph D. in science subjects like Physics, Maths, Electronics, Geology, Geophysics etc. Advertisement for such positions also given in national newspapers. Or you can also visit the official website for more information.