There are many English speakers in Costa Rica but there is definitely something to be said for learning to speak at least some basic Spanish.  Tremendous good will comes along with a pleasant greeting and basic conversation in Spanish.
Additionally, Spanish skills will help you communicate anywhere and anytime without having to seek out a translator.
There is no magic way to learn a new language.  There are many learning methodologies,
language schools and online programs available for learning Spanish. An internet search will provide links to  a number of options. One example is Duolingo, a self paced online program
that some of the native English speakers in our office have found very useful.
Starting with the basics will keep you from becoming overwhelmed. Learning simple greetings, the alphabet, pronunciation of letters and counting is an excellent way to start. Once you get started be brave and use what you know every time you have an opportunity. Locals are very appreciative when visitors make the effort to speak Spanish and tend to politely help you find a missing word or phrase.
Most importantly, have the ability to laugh at yoursel f!  It is inevitable that you will occasionally
use a word that is completely wrong. If you confidently use your best Spanish to ask for laundry soap (jabon), only to realize that you actually asked for laundry soup (sopa), enjoy a chuckle thinking of yourself pouring chicken soup into the washi
ng machine. There is a good chance that these types of experiences will result in words
that you will never forget.
Living in Costa Rica provides a great opportunity for learning Spanish with daily opportunities to practice your new skills. Maybe you will want to give it a whirl.