Just get as much Experience (or "XP") as you can! You can get XP by completing quests, finishing buildings, placing decorations, performing expansions, winning XP in the Balloon Pop game, or getting new characters. If you want to know how much XP you have as well as the amount of XP needed to reach a new level, just tap the blue bar located in the top-left area of the main game screen to access the Profile menu. While in the Profile menu, tap the blue XP bar in the top-left corner of the screen.