One of the first things you need to teach your young puppy is how to behave on a leash. It is a crucial skill to master for both you and your dog's future. After training a puppy to walk on a leash they are much safer and more manageable, plus it means walkies is an experience you can both look forward to and enjoy.
To start with just walk around the house with the leash in hand, and your puppy trotting along side you. When he is walking along on a loose leash give him plenty of praise, petting and even some treats. When he strains on the leash, stop immediately. Don't yank 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 puppy is pulling on the leash, this only rewards his behavior and reinforces the habit.
Your puppy has to learn that when he pulls on the leash, he gets nowhere. If he wants to continue walking, it has to be by your side on a loose leash. The same rule applies if your puppy sits down when you are walking. Don't yank him forward towards you, just call him over and reward him when he arrives. Then set off walking again with your puppy by your side.