Dogs walk faster than humans can. Naturally, if you put them on a leash, they'll start pulling to get to whatever they're looking at faster. A dog, having what is called an opposition reflex, would lean on a pressure it feels on its neck and chest. Understanding this makes it clear that if you want to have a leisurely walk with your dog on a leash, you must begin dog leash training while your pet is still a few weeks-old puppy.
Begin by allowing your dog time to get used to the collar around his neck. When you put it on for the first time, your puppy will scratch, roll around or try to get it off. Make sure you fasten the collar securely and don't remove it when the dog is trying to. When he finally calmed down and shows signs of being comfortable with it, you can remove the collar for a short while. This teaches the dog two important things: first, that the collar won't come off when he wants it so he'd best get used to it and second, the collar is not a permanent attachment. Check the collar regularly to make sure it isn't too tight. Your dog grows in size and you need to adjust the collar regularly.
When the dog is ready, attach a leash to the collar and allow the dog time to adjust to the leash. Allot a few minutes every day to for leash training with your puppy and have some treats handy. Your goal is to be able to walk around with your dog on a leash without him pulling at the other end. This means you should be able to stand still with your dog and walk around at your pace, not his.
To get started, fasten the collar securely around your pup's neck and attach the leash. Hold the other end securely. Try and have the dog stand still beside you. When he starts to wander too far and pulls on the leash, call the dog back to your side and reward him with a treat when he does. When he finally is able to stand still beside you for the time you wish to, you can begin walking him around. When he wanders too far to the point of pulling on the leash, call him back to your side. Pull on the leash if needed. When the dog returns to a comfortable distance - walks beside you without pulling on the leash, reward him with a treat.
Do these training exercises with a reward system for several days or weeks or until your dog learns to consistently respect the leash. When your dog is finally able to stand still beside you or walk around your house or backyard with a loose leash, reward your dog with a lot of praise, hugs or pats and some treats. He is ready to be taken out for a walk.
Remember that to condition your dog to respect the leash, focus on allowing your dog to progress forward when the leash is loose. If you allow your pet to proceed forward when the leash is tight, you're teaching him to pull.