Find the followings:
For U.S. citizens: You can buy ether (ETH) in MetaMask with the integrated Coinbase.
For everyone else: Purchase some ETH from any exchange. Coinbase is an easy place to buy them. You will then need to transfer your ETH to your MetaMask Address.
From you Coinbase account transfer some ETH to your MetaMask wallet address. You are now ready to play.