What should I do next with the Potato?

Dear reader,
One of the potatoes I had bought recently had a small cut, as a result of which I just threw it out into the garden instead of using it for making a dish. Yesterday, I discovered that a long green shoot has sprouted from it. I was thrilled. However, I do not know the next steps. Should I sow the potato underground or just allow it to remain on the surface? Please advise.