gistfile1.txt

function MYMODULE_enable() {
  // Check if our field is not already created.
  if (!field_info_field('field_myField')) {
     $field = array(
    'field_name' => 'field_myField', 
    'type' => 'text', 
);
field_create_field($field);

// Create the instance on the bundle.
$instance = array(
    'field_name' => 'field_myField', 
    'entity_type' => 'user', 
    'label' => 'My Field Name', 
    'bundle' => 'user', 
    // If you don't set the "required" property then the field wont be required by default.
    'required' => TRUE,
    'settings' => array(
       // Here you inform either or not you want this field showing up on the registration form.
        'user_register_form' => 1,
    ),
    'widget' => array(
        'type' => 'textfield',
    ), 
);
    field_create_instance($instance);
   }
}