Get value for a field from url while saving a new node
With the comments so far , I could understand that you have a content type with 10 fields, so the time so create a node you will be putting 9 fields on the node form and the 10th field has to be filled with some value from url parameter,so lets assume your site url while adding the node looks like www.sitename.com/node/add/your_content_type?field_value=xyz. 

One of the possible way can be to pick the value from url and make it as 10th field default value, an example code can go something like below.

 // Assuming 10th field as textfield.
 function mymodulename_hook_form_alter(&amp;$form, $form_state, $form_id) {
   if($form_id == "your_node_add_form_id") {
     if (is_null($form['nid']['#value'])) {
         $form['your_10th_field_machine_name'][LANGUAGE_NONE][0]['vaue'][#default_value] = $_GET['field_value'];
     }
   }
 }


Rest saving of fields will be handled by drupal itself.