The quickest way to learn any CMS is to get stuck in. Drupal has very extensive user documentation, better and more up to date than any book or manual, but I know there's so much it can be overwhelming. The first thing you need to do is to duplicate the site to make a test site, so that you can try out any modifications you make before you put them into effect on your production site (I'm using the jargon, now). If you do this online, duplicate your database as well and, in your test site, make sure you change the references to the database name to your test database name, otherwise you might make changes in your test site that could affect your production site.
You can set up your test site online, as I say, e.g., at productionsitename.com/testsitename/, but overall the best way to do it is offline. Xampp (Google it) is a package (for Linux, Window or Mac) that automatically sets up an Apache server, MySql and php (and more) on your machine, and most importantly gets them coordinated without any effort on your part. You then copy your Drupal files and database to the relevant subdirectories and hey presto. Then you can start probing, finding out which bits do what, tweaking, modifying, it's the only real way to learn a CMS.
