In case you are having trouble with installing Drupal for use with multiple websites this should help. Drupal's install.txt file that comes with the basic install package has a pretty good short explanation. This may not be enough especially if you are on a shared host. Some of the configurations may not be possible due to your shared host's default settings.
The first thing you are going to want to do is set up a database the same way you did the original Drupal site. The next thing you want to do is set up a new folder within your Drupal install for the new website. You want to do so in the /drupal/sites/ folder. For example /drupal/sites/examplesite.com/. You want to then copy over the default.settings.php from the original install to this folder, then copy this file and rename it to settings.php. There are some issues so it may help fix problems if you have both the settings.php and default.settings.php files
At this point your folders and files are prepared you can do several things to prepare your host pointing. You can either set up a symlink or just a direct point to the drupal directory. I originally had a symlink but have now just pointed all of my sub sites to the main drupal directory. The system knows how to point properly based upon the domain name, the domain name needs to match the folder name. My host has a nice user interface to do so. This step involves telling where the DNS should point on your server. Many hosts have /home/username/examplesite.com/ set by default. We want to point the site to /home/username/drupal/, assuming this is where you have placed your drupal install.
If it is not possible or one is not comfortable with changing the pointing then a symlink may be the answer. A symlink is similar to a shortcut in windows. It is used to to tell the server where the files will be stored. The files for your site must be stored within the drupal directory so the symlink is vital for this set up to work. If you have a shared host like I do and have not done this before this may not necessarily be so straightforward.
Here is a command to create a symlink:
ln -s /home/username/drupal/ /home/username/examplesite.com/
You will need to edit the username in the command above to reflect your own username and website.
With this you can utilize the /drupal/sites/all/ folder and create a module and themes directory to place centralized modules and themes. This can then be used for all of your sites within this instance of Drupal.