Generally speaking, «в» translates as "in" and «на» translates as "on". These are sometimes used differently than in English, and they're also both used in different circumstances when you mean "at" (e.g. «на работе» = at work, «в школе» = at school). It's difficult for an English speaker to know intuitively which one is used in which situation, so it's best to just learn them by rote.

Another example: «в России» = in Russia, but «на Украине» = in Ukraine (note that Ukraine is the exception to the rule here, almost all other countries use «в»).