To start with just walk around the house with the leas and your dog walking beside you. When he is walking along on a loose leash give him profusely of praise, petting and even some treats. When he strains on the leash, stop instantly; don't tug him back over to you with the leash, just call him back over to you and praise him when he comes. Never ever keep walking when your dog is dragging on the leash, this only rewards his behavior and reinforces the habit.