You have written something on a piece of paper , an audio file , a photo , an art or even a tweet , anything that is not replaceable can be converted into an NFT and sold, if you need . Example : A random image on your phone's gallery.
NFT's don't really have any value bound to it on its own and only determined only by demand and supply .
NFT transactions are all based on crypto , so you have to have a wallet set up before transacting.
opensea is a NFT marketplace .Surf around it to know how NFT's are created , sold and bought .
100 ! Peace to you !