As of May 2013, citizens of the United States, Canada and many other countries do not need visas to enter Costa Rica.
In most cases, you need a valid passport and an exit ticket (this could be the return ticket on your flight or an ongoing ticket to another country.)
Citizens of countries other than the United States, Canada and the European Economic Community should check with the nearest Costa Rican Embassy or Consulate, or even easier…
You’ll likely get what you need by clicking on the link below for information from the Costa Rican Embassy in Washington DC
Costa Rican Embassy in Washington DC
If your country is one of those with a visa requirement, then you will need to go about getting your visa several months in advance.