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.

Hi there!

Hope you are having a nice day!

I would like to inform you that the first step is that you should complete your bachelor's degree be it B.E. or B.Sc. or B.Tech. in a college. Some might say that you could work in ISRO only if you study in IIT or NIT, honestly speaking they don't know the reality or they are trying to discourage you so leave them aside. There are regular advertisement in ISRO official site of job vacancies to become a scientist and many other posts. Follow their link regularly and be updated. In the site you can your next step is that to download the syllabus of ISRO exams and compare with your college syllabus and learn them clearly and then download the previous year question papers which is available in the same site and test yourself. If you are pursuing B.E. Just be clear and understand the concepts rather than to mug up and vomit in the exam. Understanding your core concepts from your 2nd to 4th year will be perfect to crack the first round of the ISRO exams and there is no need of special coaching classes. I can assure this only for B.E. pursuing students because I am a B.E. Graduate and had applied for ISRO exams.

All the best for your future!

Thanks