No one can join NSG after class 12.
To join NSG one must do graduation .NSG require highly motivated individual.To join NSG a person should be a firstly in a defense ,police,crpf,BSF means in paramilitary.One whoes performance is good in service can apply for this with help of their commanding officer.

To know more follow this link:http://entrance-exam.net/forum/general-discussion/how-can-i-join-nsg-after-passing-12th-i-am-17-years-old-789033.html

Hope this helps you.