This video shows how to get a puppy started walking on a leash. If the puppy has never had a collar and leash on before, use the smallest possible collar and the flexi leash without the brake on.
Keep the sessions short at first. Call the puppy and encourage him to follow you as you walk. Work in an open area where there is nothing that he will get tangled up in.
Since puppies need instant reward for good behavior, the video also shows the right way to reward the puppy for responding to commands.