You cannot use U.S. Credit or ATM cards in Cuba. You can change USD at the airport Cambio, at the Bank, or at Cambios in the city - you need your passport to change money. I recommend you change a bunch at the airport and apparently the Cambios have shorter lines than the bank if you need more. We waited an hour at the Banco Central but it was a pretty Cuban experience so no regrets. Exchange rate was .96 USD per CUC (convertible peso), plus a 10% tax for USD. I brought Euros too. The exchange rate was bad, but no tax. Try not to bring USD if you don't have to - it's easier, and no "USD" exchange premium due to the poor relations between Cuba and USA.