To get the value of a node field first of all we need to get the node object.

So add the below code to your THEME_preprocess_node() function.

if (($node = \Drupal::routeMatch()->getParameter('node')) && $node instanceof \Drupal\node\NodeInterface) {

}

Now we get the node object. Then we can set the field into the variable we want to get the value.

if (($node = \Drupal::routeMatch()->getParameter('node')) && $node instanceof \Drupal\node\NodeInterface) {

     $variables['my-title'] = $node->field_my_title->value;

}