Dear Aspirant,

ISRO stands for Indian Space and Research Organisation. It has its headquarters in Bangalore, Karnataka. It is responsible for research in space and extraterrestrial phenomenons and also for aeronautical exploration. You are basically looking forward to be a part of the brainpower of ISRO and aid them in new discoveries and breakthroughs as a scientist .

Lets look at what you need to do to achieve this.

1. Firstly, in your 10+2, your subjects must have been Maths and Physics and you must have a thorough knowledge of their basics.
2. If you get into Indian Institute of Space Science and Technology, it will be easier for you to apply for the post of a scientist. Graduates of IIST with a minimum CGPA of 7.5 are directly absorbed into ISRO as Scientists/Engineers 'SC'.
3. ISRO recruits graduates and post-graduates from various colleges such as IIT, NIT and even other government and private institutions. It becomes important in such cases to have a course which is closer to ISRO's daily work. For example, the viable choices would be Aerospace Engineering, Mechanical Engineering, Radio Engineering and Engineering Physics. There are other vacancies as well.
4. If you pursue B.Tech., Then an exam called ICRB is conducted by ISRO for centralised recruitment. After clearing this exam and the interview, the candidate can be selected at the post of engineer or scientist at ISRO.
5. After the completion of M.Tech or PhD. in trades such as Geoinformatics, Remote Sensing, Instrumentation, applied Mathematics, Geophysics, Geography or any other equivalent degree in the field of Physics or Mathematics one can apply directly for the job of a scientist at ISRO. The selection consists of written exam and Interview and after clearing these sages scientist/Engineer position can be obtained in ISRO.

Hope this helps. All the best!

Naveen, There are various levels at which you can join ISRO and at various designations.
After 12th: You can get into IIST (Indian Institute of Space Science and Technology) for their Undergraduate programmes. The admission is based on JEE Advanced scores. Which means you need to have studied Physics, Chemistry and Mathematics in 10+2. You can pick any of their 4 year Bachelor programmes like B.Tech in Physical Sciences, B.Tech in Avionics and B.Tech in Aerospace Engineering, etc. Or you can also pick up their 5 year Integrated courses in Master of Science in Astronomy and AstroPhysics, Master of Technology in Earth System Science, Master of Science in Solid State Physics and Master of Science in Optical Engineering, etc.

Opportunities are amazing for academically brilliant candidates. With a CGPA of 7.5 or higher, subject to the availability of vacancies, you get placed in ISRO. Aero and Avio graduates have better prospects even outside ISRO.

After B.Tech: IIST also offers Postgraduate Programmes like:
M.Tech in Soft Computing & Machine Learning
M.Tech in RF & Microwave Engineering
M.Tech in Geo informatics
M.Tech in Optical Engineering
M.Tech in Machine Learning & Computing
M.Tech in Solid State Technology
M.Tech in VLSI & Microsystems
M.Tech in Materials Science & Technology, etc.

Opportunities depend upon ISRO Placements conducted by centralised recruitment committee.