Drupal provides a number of ways to work with multiple sites. This series gives an overview of those methods, and then teaches how to use the core multisite system. If you are interested in working with the Domain Access project instead of core multisite, you should look at the Introduction to Domain Access series.
Check your version
This collection covers a topic in Drupal 7 which may or may not be the version you're using. We're keeping this tutorial online as a courtesy to users of Drupal 7, but we consider it archived.