On day 5, there will be a scene where the player says they need to stop "playing the field" and that it's "time to get serious". You are then given a selection to choose one of the boys-- this is where you select which boy's route you will play! From there, every character has a single ending, for a total of 6 endings.
After you have completed the 6 routes, you will unlock the True Route, which can be accessed on the main menu.
I hope this helps! ^_^