Creating a ghost effect for video is quite simple, as long as you have an editing package capable of using layers and setting layer opacity.
First of all, set the camera up on a tripod and frame the shot. Shoot the scene empty (without the ghost), then shoot the scene again with the ghost actor. It's very important that both versions (takes) of the shot are framed identically, so don't move the camera at all. The background also needs to be still.
Import both takes of the shot into your editing program (Adobe Premiere in this case). Place the empty shot on the first video track and the ghost shot on the next track.
Set the opacity of the ghost shot to around 30%, or whatever setting achieves the best result.
Note: You might like to add a spooky sound effect for added impact.