I have registered a nameserver ns1.mydomain.com
Now the domains registered from core nick does not allow me to change the name servers to ns1.mydomain.com , I have come to know that i have to create a host reference at Joker or CORENIC.

Can somebody pleaseeeeeee tell the exact procedure step by step on how to do it :(

Hello
If you bought your domain from Joker.com you need to log into their web site and choose create COHO/host handles.
You can then add your two name servers to their database and from there they go out to CRSNIC/VERISIGN.

Gordon