Move Entries to the top of a structure when they're first saved

YourPlugin.php

class YourPlugin extends BasePlugin
{
    public function init()
    {
        craft()->on('entries.onSaveEntry', function(Event $event)
        {
            $isNewEntry = $event->params['isNewEntry'];

            if (!$isNewEntry)
            {
                return;
            }

            /** @var EntryModel $entry */
            $entry = $event->params['entry'];

            if ((int) $entry->sectionId !== 10)
            {
                return;
            }

            craft()->structures->prependToRoot($entry->getSection()->structureId, $entry);
        });
    }
}

Replace the 10 with the ID of the section you want this to apply to