I would start here:
Leerburg | Who Pets Your Puppy or Dog
And more can be found in post 8:
New Dog, Very Challenging
I did this after my first GSD greeted company with a low growl!! In 12 years of dogiehood I had never had that "issue" before (Bully breeds) so it was a surprise to me! In GSD world not uncommon. They are not particularly found of "strangers" on their turf!