Cuba is an island and you arrive by air. José Martí International Airport (HAV) in Havana is the main point of entry; Varadero's Juan Gualberto Gómez Airport (VRA) is the second busiest, handling most resort charter traffic. Santiago de Cuba (SCU) and Holguín (HOG) also receive international flights.
From Europe, Air France flies from Paris, Iberia and Air Europa from Madrid, Air Canada from Toronto and Montreal. From the UK there are no direct flights — you connect via Madrid, Paris, or Cancún.
From the United States, direct flights to Havana operate from Miami, Tampa, and Fort Lauderdale (American, United, Southwest, Delta). US citizens traveling to Cuba must qualify under one of twelve authorized categories — "tourism" is not among them, though cultural, educational, and people-to-people travel all qualify in practice. In reality, enforcement has been lax and most Americans visit without issue, but the rules change with US administrations so check the current status before you book.
From Latin America, flights arrive from Mexico City, Cancún, Panama City, Bogotá, and most Caribbean capitals.
Note that travelers arriving with US dollars pay a 10% exchange penalty on top of normal fees. Bring euros, Canadian dollars, or British pounds instead.