There is no way to program a Nissan key online. However, there are online resources that provide instructions for programming a key, depending on the make and model. Some current Nissan models have keys that are only made using the special equipment used by a dealer or locksmith.
Since the mid-1990s, most automobile keys have transponder chips that contain a code which allows the car to start. If a replacement key does not have this code, it does not start the vehicle. Websites like freeremoteinstructions.com explain how to reprogram a Nissan key, depending on its make and model. All dealerships and most locksmiths have the machines necessary to program the key for a fee.