This is the first of three leashes on this list, which may seem excessive, but I will explain the function of each one. This leash has the most obvious function. This will be the one that you will use when taking your dog out and about. This leash should be 4-6 feet in length and a sturdy enough material to successfully contain your dog. This leash should NOT be a retractable leash. Retractable leashes do not give you any control over your puppy, which is very dangerous if they were ever in a situation you had to remove them from quickly. I will link an article about the dangers of retractable leashes.
An inclusive checklist for bringing home a new puppy (straight from the mouth of a dog bather, vet tech student, and dog lover all in one). I linked many informational articles as to why you need these things, and what considerations you should make when choosing your new puppy supplies.