There is a veil of mystery surrounding SEO tactics. There are many modules out there that aid in improving your site's exposure. The easiest path for a beginning is to use SEO Tools. It basically gives you a list of prerequesites, and gives you some easy out of the box SEO.
To show all errors in PHP use these two functions:
If you are using a framework or CMS, a good place to put it is your settings or bootstrap file. For Drupal I recommend the settings.php file.
Google Code Prettify is great for turning <code> tags into nicely formatted HTML. I personally appreciate how it displays the line numbers, but they don't get included when you copy+paste. There are many themes available but my favorite is the google code. It treats word wrapping well with line numbers, shows each line number, and is styled nicely. The Drupal module allows you to integrate it easily to Drupal.
Drupal Pretiffy module: http://drupal.org/project/prettify
Manually alter the Drupal theme in MySQL using these commands.
UPDATE system SET status=1 WHERE name = 'garland';
UPDATE variable SET value='s:7:"garland"' WHERE name = 'theme_default';
NOTE: There are many cache tables in Drupal 7 that may need to be flushed.