You will be able to exchange money once you have arrived in Cuba either at the airport or at CADECA bureaus can exchange currency (US and Canadian dollars, Euros, British pounds, etc.) for you. Larger chain hotels are often the most convenient place to make currency exchanges. Be sure to have proper ID (passport) on hand and make sure you receive a receipt for the transaction and count your change.
Do not accept offers to exchange currency from anyone who approaches you on the street. It is illegal and a common scam practiced all over the world to take advantage of travelers, who are unfamiliar with new currencies. This is particularly applicable, due to the dual currency system used in Cuba.
Securely carrying your funds in a money belt and leaving excess funds behind in your hotel safe is also a good idea.