Puerto Rico is a commenwealth of the United States but the main language spoken is Spanish. The majority of the Puerto Ricans also speak English and are very friendly.
The only time you will actually need to know a little Spanish is if you travel outside the metropolitan area.
Make sure you know a least a minimum amount of Spanish. A lot of the locals do not speak English.
Some good phrases to know are:
Donde esta el bano? -- Where is the bathroom?
Cerveza por favor. -- A beer please.
Habla ingles? -- Do you speak English?
In the San Juan area, virtually everyone speaks English. It wouldn't hurt to know at least a little Spanish, especially if you want to see more of the country.