My sheepdog pup is 6 months old. He sometimes plays rough when gets excited and starts to pull and nip on your clothes and hands. Can you please give me some advice. Does this mean that he is a nasty dog? or isthis normal behavior for a puppy.

Sounds like normal behaviour to me.  Someone more experienced will hopefully give you some advice on how to stop it.  Usually it involves stopping play immediately and turning your back on your dog when he does this.  You may need to leave the room for a few minutes (or put the dog out) whichever works best.  From my experience with my own dog who was a terror for doing this, it took a long while before she stopped.  Sometimes a squirt of water in his face each time he does it may stop him - this didn't work with mine however, but does with some dogs.  Also, try distracting him with a toy - so each time he goes to nip you, put a toy in his mouth.

Hope this helps,

Fiona