If you want to visit the island, you need to process your visa prior to departure. The tourist visa is easy to get for any foreigner, and you should consider obtaining your visa before buying your tickets.
The tourist visa, whether for American citizens of citizens from other countries, who are not originally from Cuba or have relatives in the island, is easy to obtain and is usually offered as part of the deal when you buy your ticket through an agency.
One of the requirements to obtain your visa for Cuba is to demonstrate that you already have an official accommodation. Your accommodation must be only at one of the places that are authorized to receive international tourists.
There are 2 types of currency in Cuba. One is the CUP or Cuban peso and the CUC, or convertible Cuban peso. Most of the services are sold in CUC, especially to foreigners, but there are certain things that you can only buy by CUP; such as fruits and public transportation. Also CUP is used to buy some food offered on the streets such as ice cream, food, pizzas, sandwiches and others. 1 CUC = 1 U$, but in Cuba 10% is added as a charge to the American dollar so it is best to take Euros and get change in the island.
When arriving to the airport in Havana, just get the necessary local currency for the taxi, as you get considerably less for your money, around 1,23 CUC x each Euro. You have to change money only at the official currency agencies, they are called CADECA and they are the only ones with authorization to process money exchanges. You can find one at Sevilla Hotel and another on Obispo Street. Avoid money exchanges on the street.
When exchanging 5 CUC into CUP, they give you 24 CUP per each CUC, but not all the exchange agencies change into CUP, so you want to go to the one on Obispo Street.
A taxi fare from the airport should be 20 CUC, and it would be ideal to share the ride with another person on your same flight. This would cost the two of you 25 CUC, and the taxi driver will take you for that price to different addresses, for example one downtown and the other in Habana Vieja.