Dear Harshil, you have to visit your nearest Accredited Institution or study centre also known as Common Study Centre or Facilitation Centre or any NIOS Regional Centre for online registration. Or you can fill in the online application yourself. 12th is usually called Sr. Secondary. In order to take admission in the Senior Secondary course, you must have passed the Secondary course from a recognised Board. The minimum age to take admission in the Senior Secondary Course is 15 years as on 31st January 2019 (born on or before 31/01/2004). If you fulfill this criteria then regarding online admission send your e-mail to lsc@nios.ac.in