How do I find a template?
If you want to change the default value of fields in node edit form. You should looking on any *.tpl.php files. You have to define your own custom module and use hook_form_alter

You can use this if-statement inside the hook function.

// remember change [nodetype] to the one you want to alter.
if($form_id == 'nodetype_node_form'){
    // your code here
}