In order to use widgets, first we have to include the initialization code in our functions.php file:

<?php 
// Theme Support
function theme_setup(){
  // Featured Image Support
  add_theme_support('post-thumbnails');
  set_post_thumbnail_size(900, 600);

  // Post Format Support
  add_theme_support('post-formats', array('gallery'));
}

add_action('after_setup_theme', 'theme_setup');