Buy some ETH for gas fees

To send USD1 to anyone, or to acquire USD1 by "swapping" other cryptocurrencies (including ETH itself) for it, you will need to have some ETH in your wallet to pay for the gas fees

One simple way to acquire ETH is to click on "Buy ETH" in MetaMask:

You can look at web sites like Etherscan to get an idea of how much ETH you need to transfer ERC-20 tokens such as USD1