You have to purchase haunted items for your homes, look under featured items in the store anything with a small ghost logo beside it is haunted. The more haunted items you have the better chance of finding different ghosts. Also you have to level up your Sims by ghost hunting, each time they level up they will be able to find new ghosts. You have to get at least one Sim to level 6 to find all the ghosts, note any Sim can find it after you get at least one to the max level of 6.
After you have found all the ghosts you will be able to get a free haunted house the next time you get to create a new Sim, or you can delete an existing Sim to make a new one. The second time you hunt for the all ghosts it goes much faster you can find them all in under an hour, then you can buy the ghost tank...IMHO its lame, doesn't do anything but swirl and your Sims can stare at it. Fail. Anyway it gets good after that because then each time you reset your ghost hunt and find them all again you rack up money, town value and life points. I have almost 300 lp and 100,000 dollars from just ghost hunting.
Good luck and hth!