Start with downloading the plugin called Force Post Title.
Here's the plugin with one row (2 with the comment line) added to the bottom, based on our comments.
What happens is that a small jQuery script is added to the page Create Post/Page. The script will check if the title field is empty when the user clicks the submit button.
Since this is such a small plugin, you could easily modify it and copy it to your functions.php. That way, you won't have to edit an existing plugin which will give you a headache once you update it later on.
I should also mention that I found something (a filter) that could work in wp-includes/post.php, row 2489. I did some quick testing without any results though.
Plugin Name: Force Post Title
Plugin URI: http://appinstore.com
Description: Forces user to assign a Title to a post before publishing
Author: Jatinder Pal Singh
Author URI: http://appinstore.com/
var testervar = jQuery('[id^=\"titlediv\"]')
if (testervar.val().length < 1)
setTimeout(\"jQuery('#ajax-loading').css('visibility', 'hidden');\", 100);
alert('POST TITLE is required');
// Add this row below to get the same functionality for page creations.