
This is normal puppy behaviour, so don't worry that you've got a vicious dog! There are many previous threads on this board about this very problem, so you're not alone! If you do a search you will find lots of advice on how to cure it.
I suggest you buy "The Perfect Puppy" by Gwen Bailey, which will give you masses of advice on how to prevent problems, and how to train your pup. Good luck! :)