It's always been my experience that a dog, being a territorial creature, will naturally guard it's home. Some types, breeds, and personalities will do it earlier and more instinctively, of course, but it is "the nature of the beast," so to speak, to guard and defend home and family.
Bimmer, our German Shepherd/wolf mix, immediately challenges anyone who comes up our driveway, keeping them at bay until he is satisfied that they mean no harm or we tell him it is all right and call him off. Even then, he never lets that person out of his sight, although they may not be able to see him! (lol) One night we didn't hear my Dad drive up and he came to the back door. Bimmer was there, but kept getting in between him and the door so he couldn't open it.
Each of our Filas is different, also. Buffy, our first, just automatically barred anyone from getting out of their car until we came out and got her, and even then would glare at them balefully until they left. Shiva will challenge, then sniff, then (so far - I'm told her temperament may change drastically when she's mature) generally greets the person a bit too enthusiastically for their own comfort. Imagine a 115 pound puppy, wriggling and tail wagging and being instant friends! Kharma, at 10 weeks old, is reserved, but is still willing to be guardedly friendly, but will still bark and snark at a stranger.
All of the dogs will let the truck from the farmers' co-op unload stuff at the barn, but no one wants to try picking anything up!
My Bear would let friends walk up in the yard and play with him if we weren't home, but if they tried to go near the door, he stood across it and pulled back his lips. As soon as they went back out in the yard he was perfectly happy to go back to playing.
So, you see, different dogs have different ways they guard their homes. Your dog may already be doing it, just not in a way that you realize.
You shouldn't have any problem at all with your dog protecting your home! Dobermans are natural guardians. It's a good idea to train your dog not to take food from anyone but you, though. It's also a good idea to walk your dog around the perimeter of your yard at least once a day to define his or her territory.