Dog Behavior

Question
why my dog is skinny?
and how can I let him to be afraid of me?

Answer
More details would help.  Is it a young shepherd that isn't eating all the food you offer it?  If it will eat more, just give it more.  Recomendations on the bag are just starting points.  I once had to give a Lab 7 cups a day of Pro Plan to bring her up to the proper weight.  If that doesn't work, the best thing is to talk to a vet.  Or, you could examine his bowel movements yourself looking for white worm eggs or segments.  If you find any, try to match them to worm remedies where they sell pet supplies.  

Some Shepherds will only eat enough to stay a little on the skinny side.  Feeding a meat based concentrated chow may help.  Otherwise, compare him to the guide at http://www.puppychow.com/products/popup_body_condition.aspx  Again, a vet can confirm your judgment.  

A dog needs to respect its owner as a leader, but should never be afraid.  Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog.  Life is much easier if the 2 legged pack members outrank the 4 legged ones.  You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/  The top dog should be more of an old fashioned strict, but loving parent, not a drill sergeant.