how to customize Registration form
As an adminstrator, you can set additional user fields at:

/admin/config/people/accounts

Note the tabs Manage fields and Manage displays. You can control if and how these fields appear on the registration form, and also how they appear to the user via these tabs. You can even create non-visible fields for storing other kinds of hidden data against a user should you need to.

By default, the link to register is:

user/register

You would normally create a menu item if one doesn't exist already to appear somewhere on your site, or you can just output the link in a suitable location depending on your needs.