Basically, you need to please one of the characters depending on the route you're taking.
The routes are unlocked by being nice to specific characters, you need to lend you pen to mark to be able to choose his route, for example.
You'll decide which route you want to take when you are in your bathroom ( at the beginning of the second day, i guess ), after that you just need to be kind, gentle and remember important things they say, so you shouldn't open something that you don't have permission, don't bring unhealty food for someone that only eat healthy food, etc.
I hope it was helpful :)