When you're training your dog to not pull on a leash make sure you have them in a harness and not a regular collar.
When your dog starts to pull on the leash you should turn around in the next direction and say lets go.
You might have to tug on the leash to get the dog to turn around and that's okay if you have too.
You might have to continue to turn around and walk the other way with the dog a few times before the dog learns to not pull on the leash.