See that button that says X Present in the bottom right? Press that, and if you have the right items in your inventory (They'll state on the second item description ((which you also get by pressing X)) that they are liked by someone) and then give whatever item to them. Some items work better than others but you can give them enough crap to make them love you. You'll know it's working when a chime pops up when you talk to them and they are all blushing.
X advances the next message the NPC will say. I think you mean "Square," not "X." Press square to give the NPC a present. Give them the right item and they'll like you alot. I can't remember if there is a guide on what NPC likes what item best but I remember that Selene likes Toadstool Sitter which can be found on fallen tree gather spots.