If you don't know anything about it already, I recommend reading up on distressing clothing.
Here's a little bit on how to distress clothing for Halloween dummies:http://www.ehow.com/how_4498089_distress-clothing-halloween.html
... Ultimately, for ghosts and things, you want to have the clothing period match the time the ghost or ghoulie was alive. Also, poltergeists usually don't physically manifest. They're usually invisibly throwing things or making noises. They also are usually more predominant if the person these events happen around is a teenager. If it was a mischievous ghost, you might want to go for a youthful look. Then again, there are plenty of stories of older pranksters. I'd have to know more about the particular ghost you're going for before I could give tips as to appearance or suggestions for clothing. I'd also have to know what sort of film this is-- low-budget, home-made? Will there be special effects? The list goes on. I truly don't have enough information to provide much of a useful answer. Read your script and maybe ask the director what he/she envisioned the ghosts looking like.
I recommend renting some old scary movies and going off that-- that was before special effects and they relied a lot more on costume, makeup and cinematography. You may even be able to find information about makeup from the older movies online. In general, they do hallow eyes by blackening the area around the eye. Also, sallow cheeks were added by darkening the area under cheek bones-- somewhere near the area where women normally put rouge.
If you're looking for cheap clothing or older clothing to distress, try a second-hand store or hit up your grandma/pa or mom/dad for things out of their closets.