How to Train your Dog to Walk on a Lead
The essence of training your dog to behave while walking on a lead, is to work on socialising them and perfect the art of getting your dog to focus on you.
By training and rewarding your dog to focus and pay attention to you, they're learning that paying attention to you is more rewarding than doing other things. Things such as leaping, lunging, or just pulling on the lead.
While you're doing this training you're building a relationship with your dog, who wants to do more things with you. You should never rely on techniques that are punishing - as these types of things damage your relationship.
Step-by-Step Clicker Points
Step 1: Start in your own home or backyard, with some training treats in your hand.
Step 2: Spend some time getting and keeping your dog's attention, to show that paying attention to you is more rewarding than looking around or leaving your side.
Step 3: Take a few steps away from your dog, call their name, and reward when they come to you. Do this a few times so that you're reinforcing this good behaviour of keeping their attention focussed on you.
Step 4: Now it's time to take your dog out onto the footpath outside of the house or yard, where there's going to be a few more distractions. Cars, noise, other people etc. Walk a few steps and then stop, call their name to ensure the focus is on you, and then reward. And then repeat this for ~20 or so metres along the footpath. And then repeat this daily to build this behaviour.
Good luck!