I believe you have to have a unlocked GSM phone like att and tmobile, verizon and sprint use CDMA system, having said that, some phones have dual sim capability, they would work with both, GSM and CDMA systems (have your phone serial# ready and check with Apple). I have a unlocked GSM iPhone 5 and I bought a Movistar sim for $4 (in CR from a Movistar booth downtown, they are allover the place, just ask some one) and when there I use it mostly for texting CR only). When asked the girl at Movistar counter how many texts I can send, she told me "mucho mucho don't worry" If you are thinking about calling back to US, you will need a special plan. I use my laptop to call US numbers from CR (for free). Check with google you will need to setup a Gmail account.
When there be aware of your surroundings....Don't wonder around in a remote area, especially at night. If you have to go somewhere at night take a taxi, even if its only one block. Pura vida

ps They don't have a nano sim, some places will cut it down for you some won't.
Check these links:

http://www.bgr.in/news/how-to-cut-a-...m-to-nano-sim/

http://doingcostarica.blogspot.com/2...osta-rica.html