Once your new Drupal site is up and running, your next challenge may be to port possibly years worth of content from a previous site.
There are probably as many ways to do this as there are sites on the web. Therefore no one method works for everyone, but you usually do not need to start from scratch. Various migration methods have been contributed to the community. There are even Drupal modules to convert an entire site from other standard CMSs into Drupal.
See the page, Migrating to Drupal in the Installation Guide for more information.