IMMEDIATELY go see a bheaviorist. This is a VERY serious problem and requires professional aid. Your vet will recomend a canine behaviorist for you to see, and this person will evaluate your dog's behavior and tell you precisely what you must do to correct it.
In the meantime read this thread about dominance training:
http://www.yorkietalk.com/forums/sho...7&postcount=20
I want to stress this: Your dog is young enough that if you go RIGHT AWAY, this can be corrected easily and quickly. If you wait even a single month, you will lose some very valuable time as the 1 year mark is seen as a sort of cementing of agressive and dominant characterisitcs. Your dog is currently an adolescent and still adjustable. If your dog acts this way toward a stranger or a child he may have to be put down. This is a very serious behavior and is far more than a simple training issue.