Your puppy is too young to expect her to walk on a leash. Stay at home, put the leash on her, let her walk and drag it around the house and yard.
You can do an exercise to help her with the leash acceptance by having some yummy treats in your hand or pocket. Get her attention, walk slowly backwards calling her name, give her a treat. Do this first with her just dragging the leash. Then, begin holding the leash and give her treats as she follows you around the yard. Gradually, over time, get her used to going further and then gently, and with more treats, begin leaving the yard as she gains confidence.
If you continue to hit her in frustration, you will ruin this dog. If you can't be happy and positive when playing and training her, don't do it at all.
Sabra - female GSD - born April 30, 2012
Sargeant - rescued senior GSD male/foster failure - approximately ten years old (August 2013)
Max - rescued GSD - Dec 22, 2010 to March 7, 2012 - at the bridge
Sable - rescued GSD - at the bridge