How to get a puppy used to a leash. Put a non choke collar on the pup and under your supervision let the leash drag behind the dog. If the pup is near you pick up the leash but only for a few minutes then let it go. Keep this up until you can say "Name, Come" and gently pull the pup to you and give a treat. That's not just a leash train it's also training "Come" only say 'name, come' ONE TIME then treat even if it takes a few minutes to show up. Don't repeat the command so they learn it's IMPORTANT. You can buy soft treats or use hotdogs up but not cooked.
Once the pup ignores the leash use it in the house to walk the pup into another room. Keep doing it until the puppy doesn't mind a collar and leash. Then you're all set. The worst leash on the market is a retractable one. DO NOT USE ONE. They leave you with NO control.