Go into your MetaMask wallet. Switch the connected blockchain by clicking on the tab saying Main Ethereum Network. Scroll down until you find Custom RPC and cick on it. Or go to Settings>Networks and click on Add Network. Fill in the blank spaces as follows.
Network Name: Polygon
New RPC URL: https://polygon-rpc.com/
ChainID: 137
Symbol: MATIC
Block Explorer URL: https://polygonscan.com/
After that click Save. You have now connected to the Polygon mainnet with your Metamask Wallet. Transactions on the Polygon network require its native token MATIC which will be used as gas fee.