This howto shows you how to do a multi-site Drupal install on Ubuntu. It also covers how to layout your directories for ease of maintenance, and how to ensure that you can update Drupal easily from CVS
Read it at howtoforge
Why go to all this bother? Why not just install Drupal using sudo aptitude install drupal? The answer is that as your website gets more popular, you're going to need to keep up to date with security patches, as well as manage all the custom modules you've installed. Also, it's nice to have one website as your main site, and a second one where you can test new Drupal features, modules or ideas. And who knows, after your first successful website, you'll probably want a second and third one (I know I did), so let's make it easy from day one.