The admission in Indian Statistical Institute is given through ISI Admission test. For more information please look up the following link http://www.isical.ac.in/~admission/IsiAdmission2016/AdmissionNotice2016.pdf Thank You.
To get admission in Indian Statistical Institute, first of all you have to appear for its entrance exam and based on the performance by different candidates in the exams merit list is made through which students are selected.
So, perform well in the exam and clear the cut-off.
All the best!!