Position: Entry or outside
If you have the outdoors area to have it, a cemetery is very effective. Styrofoam headstones are good, as are wooden headstones. If you are really ambitious, or have someone really talented, look to old cemeteries for inspiration for decorations. There should be plenty of dead leaves around, and there are many electric lanterns that can mimic real lanterns quite well. (Please, please, please be very careful if you choose to use real candles.) Fake cobwebs are good, and carefully used dry ice completes the scene.
Any Halloween store can sell you props that will be quite effective, but for pure creep factor, nothing beats a little bit of applied research. Go online and find common first and last names from 14 to 20 years ago, as chances are that you will hit on some quite close to your target audience. Put dates that are just a few days in the future. A teenaged girl who, for example, sees the name of her best friend on a tombstone with a death date just a few days away is going to get freaked out— and you haven't even done anything yet.
An indoor cememtery is, of necessity, quite limited. Consider having a headstone with a face-shaped hole in it, so that an actor with white or stone makeup on can make faces at the guests. (Underlight them for the most effective mood.) Dry ice can also be used, but try asking a local theater if they have a dry ice fog machine. That's the thing that gets the fog crawling across the floor— a very disturbing image.