Let's create a new directory to work in and navigate to that directory:
$ cd ~/Desktop
$ mkdir planets
$ cd planets
Within the directory planets, we'll be making a new directory to track changes
$ git init
Now, by running $ ls -a, we should see a new subdirectory .git. .git is where all of the history and metadata for the planets project is stored. Deleting .git deletes this history.
We can see the status of our new repo with
$ git status
which returns something like
On branch master
No commits yet
nothing to commit (create/copy files and use "git add" to track)