How can I buy or sell cryptocurrencies with fiat money?

You could use a service like Coinbase to buy cryptocurrencies with dollars, euros, or other fiat currencies. If you follow this link you even get $8 worth of cryptocurrency for free: https://www.coinbase.com/join/5a0b0ab35e18250227c7c45a.

After you've bought Bitcoin or Ethereum for example, you can exchange them for another cryptocurrency on Binance: https://www.binance.com/?ref=35424440.