The gas is a small fee that you need to pay to use the Ethereum network. If you send your tokens to someone else or to an exchange, you need to use some ETH to fuel the transaction. If you execute the mint() function, you also need some gas to execute the smart contract. You can buy some ETH from a friend, an exchange or even on eBay for a small amount of ETH (seller will charge you twice the market price but it's the easiest method for a beginner).