You need a digital wallet (such as Metamask) with funds to buy or sell NFTs. On the most popular NFT marketplace, OpenSea, the currency you will use to buy is Ether (ETH). 

Check the following articles on OpenSea on how to buy and sell NFT’s.https://support.opensea.io/hc/en-us/articles/360063518033-How-do-I-buy-fixed-price-NFTs-
https://support.opensea.io/hc/en-us/articles/360063498333-How-do-I-sell-an-NFT-