I'm working with Callan now on the same thing.
As mentioned, get him used to harness or collar first.
Then clip on the leash and let pup drag it around a little. To get used to the weight.
Then with treat bag readily available, pick up leash. Treat dog.
In the beginning, keep the leash short. You want the dog close to you so you can easily reward and pup makes the connection
Take a few steps with treat in hand and in front of pup. If dog follows, then treat. Repeat, going further and further each time. It won't take long for puppy to be bouncing along beside you.
If dog struggles, stop. Put pup in a sit if possible. You want the dog relaxed. Then start again.
It takes time but suddenly one day it will just click! And then your next challenge will be pulling.
One caution. If their are a lot of distractions outside, practice indoors. One squealing set of tires on the road, will through the training off.
P.s. Any reason why you are starting with a harness?
I grant that collars can be slipped, but a harness may be too much to start with