Woah, first of all you should put a fat SPOILER warning in your post since you're talking about endings here and we don't want to spoil newcomers.
What you're looking for is called the harem ending, it's the best ending possible. There is an official walkthrough among the downloadable files, it should tell you everything you need to know.
...Or, if you don't mind replaying the game, I can offer you a mod that can nudge you in the right direction while you play. Your choice.