Your puppy has a lifetime of walks ahead of him. Walking with your dog on a leash is great exercise for you as well as for your dog. These bonding moments you share together create time for you both to exercise, enjoy some fresh air, and get out to play. Start your leash manners training as early as you can with your puppy, so the message of your expectations sinks in while he's young and stays with him for many years to come.
When you train your puppy leash manners, you are teaching him that you expect for him to be on the leash, not pulling on you, and not distracted by other things in your path. The size of your dog doesn’t matter. A small dog can pull on leashes and injure his neck. A large dog can pull on the leash, dragging you along.
Giving the skill of leash manners to your puppy will require lots of walks. Don't expect to take your puppy on one long walk and settle this in one training session. At first, consider keeping your walks slow and short, so this training remains simple. You will be showing your puppy what your expectations are by rewarding him for positive behaviors when he is doing well. You can offer your puppy redirection when he needs it, and he will, by stopping and starting over. You may need to consider starting your training on a short, tight leash and then loosening the leash as your puppy becomes better at leash manners. You can also start early training sessions with your puppy indoors instead of outdoors to keep those distractions to a minimum. Starting during puppyhood sets your little guy off for a lifetime of leash manners and fun walks.
To begin training leash manners, you will need a collar for your puppy and a leash appropriate for his size. If you have a large breed dog, you might want to consider using a harness instead of just a collar. Even though your large breed puppy is fairly small now, as he grows he's going to get larger and much stronger--potentially even stronger than you. Put in your large dog on a harness rather than a leash will give you more control. Make sure you bring lots of high-value treats to your training sessions with your puppy, so he works hard and earns fantastic rewards.