Welcome to planDisney, Gianna!
"I'd say the first thing you need is ... a pumpkin." Oh, wait! That's not quite right. The first thing you need is the Disneyland mobile app. Just search for Bibbidi Bobbidi Boutique or find it on the map under the shops. Tap your magic wand, er, finger on the "Check Availability" button, and "bibbidi bobbidi boo!" You've arrived at the ball. Well at least at the screen where you can select from available dates and times for upcoming reservations for magical makeovers of Guests ages 3 to 12 performed by none other than the Fairy Godmother's apprentices.
You may also visit the shop in Fantasyland in person to see if they have same-day availability for walk-up appointments. But your best bet is to book a reservation in advance.
Tony's Tip: My daughter thoroughly enjoyed the royal pampering she received at the Bibbidi Bobbidi Boutique. We found that the perfect "happily ever after" ending to her transformation story was to find a Disney PhotoPass photographer to capture her special day forever. Digital downloads of PhotoPass photos are complimentary with a purchase of Lightning Lane Multi Pass, which might also be convenient if your little prince or princess should wish to enjoy some attractions featuring Lightning Lane entrances for shorter wait times.
A dream is a wish your heart makes. And I hope your dreams come true, Gianna.
~Tony