How To Add Field In Registration Page
You can either use Profile2, or you can add your fields in /admin/config/people/accounts/fields.

From Profile2 project page:


  Designed to be the successor of the core profile module, which is
  deprecated for Drupal 7. In
  contrast to the deprecated module this module provides a new,
  fieldable 'profile' entity - leverage the power of fields!


You can find out how to create profiles in Profile2 documentation