What preparation should I do to get admission into NSD (National School of Drama)? And what is the procedure to get into it?

There are one-month-long theater workshops held for children (age up to 18 years) during summer vacations (May-June). You will have to audition for it. Once you are selected you can enjoy the theater workshop and join Sunday Club part-1 later. Only people who have done the workshop, audition for it. After culmination plays and improvisations, you can further join Sunday club part-2 by auditioning. 

NSD offers various courses and workshops for various age groups. They publish articles related to admission and plays from time to time.