If they sing click them, If they get the angry speech bubble click them, if they have the scribbles in a speech bubble use the soccer ball.
Every Digimon also has a favourite food (and hated food), Friendship goes up a decent amount feeding them their favorite food (The icon will get a star next to it when you find it's favourite)
Later in the game after you recruit more digimon to the city there are some food you can cook that will increase friendship easily.