Building A Ghost
There are three different ways to build a ghost. You could look through the various Numina lists (World of Darkness core rulebook and Book of Spirits predominately) and then come up with creepy uses of existing Numina. You could come up with a backstory and figure out what type of ghost might be left behind and what their unfinished business might well be. You could figure out their role in the plot and determine what you would like them to have as well as what sort of Numina would be applicable.
You also need to know what sub-genre you will be using for your horror story. A tragic tale of misery and mystery will be undermined by a heavy hitting ghost that materialises to howl in your face and then belt you across the room. Something subtle would be more fitting there. On the other hand an action adventure where the ghost is little more than a mild telekinetic force that flings the occasional book isn't very interesting on it's own.
Also think a bit about what ghosts represent in your world. If they only the souls of evildoers and/or victims of terrible crimes they will be quite different than if the gentle ancestors of the past can also roam the night. Depending on your mythos, ghosts might be capable of fully materialising like normal human beings, be invisible to all who don't have the right sight, or be forever visible but unable to touch the world around them. They might be able to wander or be anchored to a single point. It might even vary between ghosts. Go with wherever the story takes you.
You may want to consider increasing their attributes as well. When you have no skills to modify your attributes (assuming the ghost you're creating has no skills), then you really need a higher Power, Finesse and Resistant stat to get anything done. Most ghosts are meant to have between 1 - 5 in those attributes with anything higher reserved for the truly powerful. Give the ghost what it needs to be a reasonable threat or an interesting ally.
There is nothing to say you have to rely on the Numina you have in front of you. You can feel free to make your own. You could even treat ghosts as a series of magical traps, really, with a dice pool listed by every event. Enter the living room and the fireplace erupts for Power + Finesse dice with a chance of setting alight any player character who fails a Dexterity + Athletics roll. Then if you go into the kitchen you must cope with a nauseating stench (Power verus Composure roll) that might well levy a -2 penalty, and so on and so forth. This can actually work out better more often than not as it feels more natural.
Naturally I'm going to include a few more of the violent sorts of ghosts that are a bit unusual since you can largely create your own haunting patterns or regular ghosts as you please.
The myling is an infact who died after a life without knowing the love of a caring parent and whose bodies were left without a proper burial. Those who cross the woods at night might well hear the Myling mewling and moaning and if they search for the sound of the babies cries they will find a baby in the woods. Mylings are filled with pain, frustration, coldness and hunger
Mylings are thought enormous and apparently grow heavier as they near the graveyard, to the point where any person carrying one (or more) could sink into the soil. If one should prove unable to make it into the cemetery, the myling kills its victim in rage. If the person who picks it up has an unclean heart (Morality 5 or lower), the Myling inflates to an enormous size and attempts to crush or rend apart it's victim (brawl attacks on a grappled target).
If picked up by someone with a clean heart (Morality 6 or higher), it seems to weigh down on the person carrying it as though the burden of its sorrow, frustration and misery presses down on the person's shoulders and the ground before him is like a bog that he slowly sinks into. The carrier must roll repeated Resolve + Composure checks to carry on and for every failure he sinks half a foot into "bog" before him. He must make such a roll 10 times before he will reach his destination and he cannot put his burden down nor can it be wrested away from him without taking the skin off his hands as well (two aggravated damage).
Power 7, Finesse 5, Resistance 6, Size 2 - 10. Numina: Banshee Wail (child-like), Drain, Morality Crush (read description).
Rusalkas are ghosts formed by those who died by drowning and now haunt their resting place as beautiful versions of themselves with long green hair like floating weeds and pale skin. Such ghosts can only be seen after dark, often spotted dancing beneath the moon, sometimes calling out watchers by name to lead them into the water to drown. Some are known as river-women, most often killed by lovers. Either way the ghostly entity is anchored to the body of water where they died. They have the same racial blessings as a Nixie though they were rarely Nixies in life.
Power 3, Finesse 7, Resistance 3, Size 5. Numina: Call Water, Camouflage, Choke, Deluge.
A good list of numina can be found here.