Because most NFTs are deployed on the Ethereum blockchain, the first step to buying an NFT would be acquiring some ETH on a cryptocurrency exchange. Keep in mind that the use of ETH is only generally popular, but some marketplaces (such as Magic Eden) accept other cryptocurrencies like SOL for the purchase of NFTs. Platforms like Aavegotchi that are built on the Polygon blockchain accept MATIC as the recognized cryptocurrency for the purchase of NFTs. You can purchase any of these cryptocurrencies on a crypto exchange.