At first you need to implement hook_node_presave($node) , so it reacts when a node is going to be saved , then you have to check if the node type is the proper node type . then setting field value , so this is some code snippet :

function mymodule_node_presave($node) {
  if($node->type == "Your Node Type") {
    dsm($node);//it will show the structure of node
    $node->field_alternative_titles_title[$node->language][0]['value'] = "data";//check the node structure,this code wont work for sure
  }
}

of-course this is just an snippet , to make it work you have to know the structure of your content type , Devel module will help you , install and enable it so dsm function will work.

At first you need to implement hook_node_presave($node)

Act on a node being inserted or updated.

This hook is invoked from node_save() before the node is saved to the database. Parameters

$node: The node that is being inserted or updated.

So it reacts when a node is going to be saved , then you have to check if the node type is the proper node type . then setting field value , so this is some code snippet :

function mymodule_node_presave($node) {
  if($node->type == "Your Node Type") {
      //dsm($node);//it will show the structure of node
      // as example
      $data = $node->title.'-'.drupal_get_path_alias('node/'$node->nid);
      $node->field_alternative_titles_title[$node->language][0]['value'] = "data";//check the node structure,this code wont work for sure
  }
}

of-course this is just an snippet , to make it work you have to know the structure of your content type , Devel module will help you , install and enable it so dsm function will work.