TL;DR: 1- Have your MetaMask wallet enabled on your browser. 2- Go to QuickSwap and connect your wallet from the link in the upper right corner. 3- Click on Switch to MATIC in the same corner. 4- Approve adding MATIC network on MetaMask.
Full:
Polygon (MATIC) is growing exponentially, to get on board you need a MATIC wallet.
The good news is that if you already have Ethereum or Binance Chain wallet on MetaMask, you can activate MATIC on the same address.
If you are completely new to MetaMask, you can download it for your Chrome/Firefox from https://metamask.io and create a new wallet or import your current wallet from a seed.
Now on your browser, go to https://quickswap.exchange , which is one of the biggest DeFi exchanges supporting Polygon/MATIC at the moment. Connect your wallet from the link in the upper right corner of the website.
Now simply click on "Switch to MATIC" on QuickSwap (upper right corner, appears after you connect the wallet) and just approve the switch on METAMASK.
Now MetaMask is showing the contents of your wallet on the Polygon (MATIC) network, to switch back to Ethereum or Binance Chain, simply switch the network from METAMASK.