[support] How/where should I start with Drupal

Hi All, I'm a newbee with Drupal. I need to make some changes to web sites written in Drupal. I do have access to FTP server and to the database. I can't figure out the file structure, I don't know how to start at all. What do I need to install? How do I find the files? Where to start? Any help is greatly appreciated. Best, Katy