Hi Aravindh!
The eligibility criteria for getting admission in ISRO:-

• BE/B.Tech or equivalent qualification in first class with an aggregate minimum of 65% marks (average of all semesters for which results are available) or CGPA 6.84 on a ten scale. Minimum of 65% marks or CGPA 6.84 in section B alone for candidates with AMIE/Grad IETE qualification. Candidates who are going to complete the above course in the academic year 2015-16 are also eligible to apply provided final Degree is available by 31/8/2016.
• Age Limit: 35 years as on 25/05/2016. Ex-Servicemen and Persons with Disabilities are eligible for age relaxation as per Govt. of India orders.
• How to apply : The application for on-line registration will be hosted on the ISRO web-site between 05/05/2016 and 25/05/2016. Candidates may visit our web-site at http://www.isro.gov.in to register their applications on-line between 05/05/2016 and 25/05/2016. Applications will be received on-line only.
• Upon registration, applicants will be provided with an on-line Registration Number, which should be carefully preserved for future reference. E-mail ID of the applicant will have to be given in the application compulsorily. The on-line application has to be invariably followed-up with a 'No Objection Certificate' from the employer concerned, by those already in employment under Central/State Government, Public Sector Undertakings or Autonomous Bodies, duly indicating the name and Reg. No. on the reverse.
• Payment of Application Fee : There will be an Application Fee of Rs. 100/- (Rupees One Hundred Only) for each application. After registration of application on-line, the candidates have to down-load and print the personalized payment challan form, in triplicate, from the ISRO system which will contain Name of the candidate, Registration Number, Receipt Account Number, Advertisement No., and Post code. The candidates may visit the nearest branch of State Bank of India for remittance of application fee to Receipt Account (32034064593), by using the personalized payment challan of State Bank of India on any day between 05/05/2016 and 26/05/2016.
• The candidates may down-load and print the personalized payment challan from the ISRO system on any day between 05/05/2016 and 25/05/2016 and remit the application fee before the prescribed last date i.e, 26/05/2016. Upon remittance of the application fee, one copy of the challan form will be retained by State Bank [Bank copy] and remaining two parts will be given to the candidate. Out of the two challan parts, the candidate has to send one part of the challan form [ISRO copy] to Sr. Administrative Officer(ICRB), ISRO Headquarters, Antariksh Bhavan, New BEL Road, Bengaluru - 560094 by "ORDINARY POST ONLY" superscribing on the envelope "ICRB - RECRUITMENT OF SCIENTIST/ENGINEER 'SC'" within 7 days of filing of application and in any case not later than 01/06/2016. The remaining copy of challan form [Applicant copy] shall be retained by the candidate for future reference. The payment shall be made through cash deposit only. Payment of Application fee in any other form like cash, cheque, draft, money order, IPO, etc, are not allowed. Applications in respect of which copy of challan form, after remittance of prescribed application fee [ISRO copy] is not received in ISRO HQ, Bengaluru on or before 01/06/2016, will not be considered for further processing under any circumstances.

I hope this helped you understand the admission guidelines and Processes in a precise way. For more information you can check the link below :-
http://www.engineering.careers360.com/articles/isro-recruitment