How to teach a dog to walk on a leash?

I have a 6 month old German Sheperd.Her name is Shayla but everytime i get a leash on her and try walking her she pulls back and just sits.so i pull it and sometimes her collar falls off.I Tryed coaxing her but he still wont work.I also been putting a leash on her and let her drag the leash around the house.But she chews on the leash.And get its caught under a chair,on chords.And everything.Help i really wanna walk her. :/