Stop German Shepherd Dog
Aggression

German Shepherd Dogs are 'pack' animals and are very loyal and protective towards their owner, his family and property. Therefore, it is a natural instinct for your German Shepherd to be on his guard and alert at all times - so yes he will bark at intruders (e.g. when someone comes to your door) and be wary of 'strangers' passing by your house.

They bark to let you know that someone is 'invading their territory' and to 'warn off' potential strangers, this is quite natural and if you are taking the position of Alpha dog i.e. you are the leader of your pack, then it is up to you to command your dog to be quiet and restrain his behavior (i.e. let him know you are aware  of the stranger and it's OK now).

You may have a German Shepherd that might become aggressive towards other dogs,  it could be because your dog is nervous or even fearful of other dogs, or is just trying to be dominant over other dogs, either way this needs to be dealt with promptly.

The best way of dealing with this is to make sure that your German Shepherd puppy is socialized with other older dogs, let him play with them so that he can learn to respect older dogs and behave with them in a good manner, and becomes relaxed around other dogs and does not view them as a threat to either himself or to you as his owner.

You must be aware that your German Shepherd is very intelligent, and unbeknown to you, he will be watching you and takes note of your reactions and behavior.So if another dog approaches you when you are taking him out for a walk, and you appear nervous - your German Shepherd Dog will immediately be on his guard and alert because he senses that you are nervous or afraid.

It would never occur to him that you are wary of his reaction to the other dog approaching you, and his natural instinct is to defend you - so as the other dog approaches, you must remain quite casual and calm, and get your German Shepherds Dogs attention in order to distract him from the other approaching dog.

Give him the command to 'sit' and keep his attention - talk to him, give him a treat if you like, but distract him from the other dog, show him you are not worried and he will relax - the other dog passes - and the situation is now resolved.

Repetition is the key, keep talking to him and relax, then this problem with other dogs will disappear as he learns not to worry about other dogs.

This is really all about training your German Shepherd Dog, forming a bond between you, and enjoying a good, stable relationship of trust, understanding, and discipline i.e. you must be the Boss!

Do not ever tolerate bad behavior from your German Shepherd, he should never be allowed to get away with it, deal with it immediately. If you treat your German Shepherd in a fair but strict manner, believe me - he will respect you for it as the Alpha Dog i.e. his leader of your pack.

The number one reason that dogs mis-behave and don’t listen to their owners is because they think they can get away with it and won't get into trouble.

check this out for German Shepherd Training:
Obedience Training for German Shepherd Dogs