A quick google turned up lots of useful looking resources but this probably looked like one of the most useful. Keep in mind that the current TKL stable is Ubuntu 8.04/Hardy server under the hood.

TKL have a relatively recent version of Drupal in their repos. So theoretically it may be really easy to do. In practice things are often not that easy. First of all back everything up (if you haven't already now may be a great time to try TKLBAM). The try installing via apt, ie

apt-get update && apt-get install drupal6

From there I'm not really sure how it will go. I have no experience with Drupal so can't even guess. Best case scenario it will all just work no worries. Worst case scenario it will break everything and neither the LAMP nor the Drupal will work properly. Most likely IMO is that Drupal will work but will break access to your current LAMP content and you'll have to work out how to access it again. The link I provided above should be able to help you get Drupal sorted. Then I reckon you'll have to look at adding a virtual host in apache (although I'm only guessing).

Let us know how you go and any problems you hit. I'll try to help FWIW. :)

Ok. I found this: http://ubuntuforums.org/showthread.php?t=1351282 for start.

"There are a couple of places to look, but most likly under /etc/apache2/sites-availible there should be a config file called drupal. Edit the documentroot line to reflect the drupal6 folder. There may also be a default config file there, you can try editing that one too"