Github “fatal: remote origin already exists”

As the error message indicates, there is already a remote configured with the same name. So you can either add the new remote with a different name or update the existing one if you don't need it:
To add a new remote, called for example github instead of origin (which obviously already exists in your system), do the following:

$ git remote add github git@github.com:ppreyer/first_app.git

Remember though, everywhere in the tutorial you see "origin" you should replace it with "github". For example $ git push origin master should now be $ git push github master.
However, if you want to see what that origin which already exists is, you can do a $ git remote -v. If you think this is there by some error, you can update it like so:

$ git remote set-url origin git@github.com:ppreyer/first_app.git