To buy and sell cryptocurrency, you must first open an account with a licensed cryptocurrency exchange such as Coinbase, Bittrex, Kraken, Bitstamp, or Poloniex. Once you have established an account on one of these exchanges, you will need to fund your account with USD, EUR, or another supported currency. After setting up your account, you will be able to make deposits in your chosen currency and purchase or sell any supported digital currencies. You will also be able to withdraw your funds from the exchange in your desired fiat currency. Please note that due to the volatile nature of cryptocurrencies, prices can fluctuate dramatically and you should always do your own research before entering into any transactions.