Let him walk around in your back yard (or in an enclosed space like a dog park) with the leash clipped onto his collar dragging on the ground he should get used to it then if you got him from a pet store he probably came from a puppy mill and those dogs can be very sick and they have no experience with the outside world you need to teach him everything about the world as soon as possible so he's not a scared dog the rest of his life

Let him drag the leash behind him in the apartment to begin with - pick him up when you need to take him out and then pick him up again if he doesn't want to go where he needs to - pulling on the leash at this point is counter-productive.

You have a LONG road ahead of you - your pup started his life in a puppymill or with an uneducated breeder (the only kind that sell to pet stores) and in addition to having lousy genetics from his iffy parents probably lacked many things vital to his health and proper physical and mental development in those first few weeks. When he was 5-6 weeks old he was packaged up with hundreds of other pups and sold to a puppy dealer - many of the pups crammed into his crate probably died in the several days of extreme temperatures, trapped with their filth and without adequate food, water, light, or ventilation. Then he spent the next almost four months of his life being unable to properly stretch his legs and unable to defecate away from his living area (house training pet store puppies can be a nightmare)- he may have been poked, hit, squeezed, and dropped by dozens or hundreds of hands which may cause future phobias and aggression. He hasn't ever been on a leash before, probably has never been in the grass before, has never seen the inside of a house or an apartment.

Not to discourage you, but you should know that you have the equivalent of a foster kid with a history of abuse, abandonment, and neglect and there will almost certainly be issues relating to it in his future. My biggest recommendation is that you find a local puppy kindergarten/basic obedience class and have the professional trainer(s) there to help you make him into the best dog that he can be and effectively identify and correct problems before they become out of control or incurable.

Good luck!

Use treats to bribe him... And start slow, show him the leash and give him a small treat. Eventually put the leash on and give him a treat then bribe him to walk a few steps for a treat ( but don't pull him or let him pull you) eventually he will learn but it might take some time. So be patient and remember try to make things a positive rather than a negative.

You need a friend with an incentive (dog biscuits/liver treats).

Get your friend to stand at a distance of about 2-3 yards and show your pup what they have.

He should move and forget about the leash. Try this a few times until he is used to moving around with the leash attached.

If your pup knows and likes the friend this will be even better.

Get him used to it slowly inside the house first where he may feel more safe, then move outside.

Just start slow and be patient. He just needs to feel secure as he probably has never been on a leash before.

This happened to us and it's tough work because you have to seem hostile at first by dragging them and when they do start to walk you praise him in a nice tone and do it on a 1 or 2 times a day everyday at app. The same time and gradually he'll start to work