Correct procedure is to get one through Nissan.
They will need the car when it arrives, to remove the lost key from the memory, and activate the new key.
This is very expensive, but your insurance won't pay out if the car is stolen and there is an active key missing.
(There was a case of spare keys going "missing" at a dealership, and the cars being stolen shortly after purchase, turned out someone was selling the spare and the purchasers address)
The cheapskate way is to take your key to Timpsons or simillar, and they cut you a key and clone the chip for about £30.
Not much use to you, but never buy a car with only one key. Another reason is some cars (eg Toyota) have master and slave keys, and the master key is needed to activate the key programming sequence, new computer required without it).