Do a search on the internet for articles relating to aggression. There are many site with very good information. Do not yell at or scold the dog. This will only make it worse. Normal voice without excitement is required.

In part from the animal health channel

"Dogs are instinctually territorial, but they usually protect their space by marking and posturing, rather than threats and violence. Dogs that growl, snarl, and lunge without first posturing, staring, and waiting are exhibiting territorial aggression.

To successfully treat protective and territorial aggression, it is important to accurately determine the precise situations that lead to the behavior.

Treatment
Avoid all situations that elicit the aggressive behavior. If you cannot answer the door without the dog barking and growling, and the dog does not respond instantly to a verbal command to stop the behavior, put the dog behind a closed door or in its crate before answering the door.

If the dog growls or lunges, say “No” and banish the dog from the situation until it is calm and responds to verbal commands to sit and stay. Removing the dog from attention and control for attention is one of the most effective and safest disciplinary actions.

Do not use physical punishment.

Avoid sudden arm gestures or motions that dogs with protective aggression often perceive as threats.

Dogs with that exhibit territorial aggression should be placed behind a secure door when any unknown person (e.g., repair person, delivery person) comes to the home and should never be loose outside or fenced into an area that a person may have access to.

If the dog protects an area within the house, change the area in which you keep it frequently."


Here is a very good link were I get information occasionally.

http://www.dogsbestfriendtraining.com/home.php