To start catching ghosts, you must first beat the ghost hunting quest, which begins around level 23. This achievement requires you to catch all 12 types of ghost, and you will need at least one experienced ghost hunter to get them. Lower level, or beginner ghost hunters can find any that have been unlocked by your pro. The best way to go about this is to build a large room, and fill it with all of the haunted stuff you can find. In the Store, haunted items are identified by the red ghost symbol. Click on any of your haunted items, and a ghost will appear. To catch the ghost, you have to touch the ghost three times to capture it. The items have a reload time, so the more items you have, and the more hunters you have, the more time you have to catch ghosts. It seems to be pretty random which ghosts spawn and when, but keep at it. You will get some good Simoleon and town value bonuses on the way.