When you're walking your dog and it starts to pull on the leash gently pull the dog back towards you and stop and tell them to sit down.
If they obey the command then give them a treat. You may have to do this for awhile until the dog stops pulling on the leash.
Every time the dog starts pulling on the leash when walking stop walking and once again gently pull the dog back towards you with the leash and tell them not to pull on the leash.
Another way is to just stop walking and then wait until the dogs stops trying to pull on the leash and repeat the same thing over and over until the dog learns that if they pull on the leash you'll stop moving and walking.