<% provide(:title, 'Sign Up') %>
<h1>Formulario de Registro</h1>

<div class="row">
  <div class="col-md-6 col-md-offset-3">
    <%= form_for @user, url: {action: "create"} do |f| %>
      <%= f.label :username, "Usuario:" %>
      <%= f.text_field :username %>

      <%= f.label :dni_number, "Cédula:" %>
      <%= f.text_field :dni_number %>

      <%= f.label :first_name, "Nombres:" %>
      <%= f.text_field :first_name %>

      <%= f.label :last_name, "Apellido" %>
      <%= f.text_field :last_name %>

      <%= f.label :birth_date, "Nacido en:" %>
      <%= f.text_field :birth_date, 'data-provide' => 'datepicker' %>

      <%= f.label :email, "Email:" %>
      <%= f.email_field :email %>

      <%= f.label :phone, "Teléfono:" %>
      <%= f.text_field :phone %>

      <%= f.label :password, "Contraseña" %>
      <%= f.password_field :password %>

      <%= f.label :password_confirmation, "Confirmación" %>
      <%= f.password_field :password_confirmation %>

      <%= f.submit "Crear mi cuenta", class: "btn btn-primary" %>
    <% end %>
  </div>
</div>

<% provide(:title, 'Sign Up') %>
<h1>Formulario de Registro</h1>

<div class="row">
  <div class="col-md-6 col-md-offset-3">
    <%= form_for @user, url: {action: "create"} do |f| %>
      <%= f.label :username, "Usuario:" %>
      <%= f.text_field :username %>

      <%= f.label :identity, "Cédula:" %>
      <%= f.text_field :identity %>

      <%= f.label :first_name, "Nombres:" %>
      <%= f.text_field :first_name %>

      <%= f.label :last_name, "Apellido" %>
      <%= f.text_field :last_name %>

      <%= f.label :birth_date, "Nacido en:" %>
      <%= f.text_field :birth_date, 'data-provide' => 'datepicker' %>

      <%= f.label :email, "Email:" %>
      <%= f.email_field :email %>

      <%= f.label :password, "Contraseña" %>
      <%= f.password_field :password %>

      <%= f.label :password_confirmation, "Confirmación" %>
      <%= f.password_field :password_confirmation %>

      <%= f.submit "Crear mi cuenta", class: "btn btn-primary" %>
    <% end %>
  </div>
</div>

Modify fields naming to fit with user model.
Modify fields naming to fit with user model.
