what filters do I use to make a voice sound like a ghost

Depends on the ghost. Some ghosts are bigger than others.

Is it a big evil one or tiny, cute and a bit rubbish at haunting stuff like Casper?

What do you want it to sound like?

Is everyone supposed to flee in terror because it has a conquistadore costume on and carries it's own head in it's hands?

Or a daft fluffy one that everyone will laugh at and tell it to bugger off?