You didn't specify which version you're talking about, but "my tamagotchi forever" is one of the tags, so I assume you're referring to the app? In that case, you need to feed your teen sweets/meat to evolve it into Lovelitchi. The Tamagotchi Wiki shows how to obtain all the characters if you're interested in taking a look: https://tamagotchi.wikia.com/wiki/My_Tamagotchi_Forever