How do I put an existing drupal site into git version control system.

I have begun reading documentation for git, because I need to start using a version control system for a site that I am working on. One thing that I haven't run into yet and which makes me nervous is, is there a way to put an existing drupal site into a git system, with a staging area and everything. I kind of understand how to start up a new repository and stuff, but not how to nudge a pre-existing site into one...

Becky