This page explains how to solve the quest "Nat's Pop-Star Mania" in Yo-kai Watch.
Start the Quest
Go into the Hot Springs in Blossom Heights during the day and talk to Alex to start the quest.
Talk to Nat!
Go down to the Everymart in Blossom Heights and talk to Nat there.
Meg will help too!
Find Nat's friend Meg and talk to her.
Get a Yo-kai to help!
You need to get a D'wanna medal to continue the quest. You can find D'wanna in the Old Mansion in Blossom Heights. Its favorite food is Hamburgers, so you might want to go to the Nom Burger in Downtown Springdale to buy some burgers first. Then when you encounter a D'wanna, before you tap Fight in the middle, tap Item, tap a hamburger, and then tap "Use on Foe" before you go into battle. You will throw a hamburger at D'wanna before attacking. This will slightly increase the chance that D'wanna will befriend you after the battle.
Quest Complete!
After you have a D'wanna medal, talk to Nat outside the Blossom Heights Everymart to complete the quest.