This achievement is unlocked early during Day Two of the story, once you are at Lukas' house. You will be given a choice, and this will determine whether the proper action is taken for the achievement to be unlocked. The name of the achievement should be a hint toward which is the action to take (see spoiler below).
*** Spoiler - click to reveal ***When you enter Lukas' room and you are given the opportunity to check out Lukas' computer, select the option that says you are curious. This will reveal that Lukas had been perusing some risqué content. Rin and Lukas will tease you about venturing into this new world of "enlightenment". The achievement will unlock after Lukas finishes teasing you.