Shadowland by Meg Cabot

Description from Goodreads: "Suze is a mediator - a liaison between the living and the dead. In other words, she sees dead people. And they won't leave her alone until she helps them resolve their unfinished business with the living. But Jesse, the hot ghost haunting her bedroom, doesn't seem to need her help. Which is a relief, because Suze has just moved to sunny California and plans to start fresh, with trips to the mall instead of the cemetery, and surfing instead of spectral visitations. But the very first day at her new school, Suze realizes it's not that easy. There's a ghost with revenge on her mind ... and Suze happens to be in the way."

Read about how Meg Cabot got the idea for The Mediator series.