Depends on who you're going for! Special characters have special conditions to get into relationships with them. I think there's currently a way to date the best friend, classroom harasser, and roommate/roommate's partner, but no one else.
If you're talking about a rando? Some of them just may not be into a serious relationship and prefer to be fuckbuddies, others prefer open or poly relationships, others still might want a Dom/Sub relationship. But in general just date them a bunch. Eventually you can approach them on the People menu and propose a change in relationship. Or they may bring it up themselves after a successful date. This can take a looong time, like a month or more of ingame time.
