A new investor of crypto/NFT usually does not have decentralized wallets and need to transfer cryptocurrency from the exchange to Metamask.
First, you must to get your wallet address, open the Metamask small window, move the mouse to the red box and click to copy the wallet address
Take Binance as an example. After logging in, click "Wallet" and then click "Withdraw"
Choose "ETH" as the currency, fill in the address you just copied, and click "Transfer". After some confirmation steps, and wait for some time, you can click on Metamask to confirm the payment is received after receiving the notification of successful transfer!
Now you can start to mint NFT!