Customising the default "Submitted by…" text used in Drupal 7 is pretty simple, and just required a single edit to your theme's template file.
Go to your ./themes/yourtheme folder and open the template.php file in your text editor. Add the function below at the end of the template.php file (remember to replace "yourthemename" with the actual name of your theme).
$vars['submitted'] = t('Posted by !username on !datetime',
'!username' => $vars['name'],
'!datetime' => $vars['date'],