I have a 13 week old jack russell she has started to growl and snap at visitors at the door or people that stop to say hello a couple of weeks ago she would go to anyone but now she growls can anybody advise me of how to nip it in the bud

A good purchase for you would be Gwen Bailey's puppy for life training pack, it's got loads of useful tips with a video and cassette, if you put in google Gwen Bailey it should come up. 

Warm regards susan