Hi,
There are mainly 3 ways in which you can get an internship first being that you have to send them an email directly with your resume and an enclosed letter mentioning as to why you want to be a part of that internship. Sometimes contacts can also get you an internship in ISRO if you are able to find someone from your relatives who work or is connected with ISRO so that they can recommend your name. Thirdly, you can talk to your college authorities or someone from your training and placement cell who in turn might speak on your behalf in ISRO.