If you want to make a ghost you will have to kill your sim to do that there is a few ways
and to provent it!
( in the cheats below it will say: testingcheatsenabled this is cheat mode to enter this:
type: CTRL + SHIFT + C
Turn the cheat on: testingcheatsenabled true then press enter
Turn the cheat off: testingcheatsenabled false then press enter
This applies to both expansions and the Xbox 360, and PS3  console versions of the game.
Preventing death
There are a few ways to prevent death in The Sims 3.
Turning aging off will prevent all Sims from dying of old age (without
testingcheatsenabled).
If aging is on, eating a life fruit will add 1 day to your Sims life span.
Eating Ambrosia will reset the amount of days in your current days until aging.
Having a Death Blossom in your inventory will give you a second chance at life. This does
not work on Sims dying of old age.
The Brave trait will reduce the chance you will burn to death.
The Daredevil trait will reduce the chance you will burn to death.
The Handy trait will make your Sim immune to electrocution.
The Hydrophobic trait will reduce the chance you will drown to death.
The Loser trait will have the grim reaper resurrect you the first time you die. This is
not 100% of the time.
The Natural Cook trait will reduce the chance you will burn to death.
The Vegetarian trait will add days to your life time.
Creating Yellow Electricity Ghosts
Yellow ghosts are Sims that died by electrocution. This one takes a bit more setup than
many of the others. You must have a room filled with broken plumbing (sinks, bathtubs
etc.) so that there is water all over the floor. In the house you use to kill the victim
Sim, you should have another Sim with the Technophobe trait. Have the Technophobe sabotage
a large stereo, then use build mode to put the stereo in the room with all the puddles.
Now have the victim Sim go in and try to repair the stereo. After a very short while he
will get shocked and will stop repairing the electronics. Make them continue repairing the
stereo and they will be shocked a second time and die. Yellow ghosts have constant
electricity going through them.
Creating Red Fire Ghosts
Red ghosts are Sims that died by fire. Build a small room with a fireplace. Cover the
floor around the fireplace with multiple rugs and direct your Sim inside the room. Delete
the door. Start a fire in the fireplace. Before long, the room goes up in flames and the
Sim is trapped inside. The fire will eventually put itself out, once all the rugs have
burned up. Red ghosts coloring moves similar to flames and smoke is coming out of them.
reating Purple/Pink Starvation Ghosts
The purple/pink ghost is a Sim who starved to death, the quickest way to do this is use
the testingcheatsenabled cheat. Once you turn the cheat on hold shift, left click the
hunger meter and drag it all the way to the left. The Sim will die of starvation and will
have a purple/pink smokey look to them. If you want to do it the long way, just wall them
up in a room or turn free will off. Dont let them eat, and within a couple days theyll die
and turn into a purple ghost.
Creating Blue Water Ghosts
The blue water ghost is a Sim who drowned to death, the quickest way to do this is to
build a small pool (33) and tell the Sim to swim in it. Once they are in, build walls
around the pool and use the testingcheatsenabled cheat, hold shift and drag their energy
meter to the left. this will make them drown and their ghost will be blue and watery.
Creating Gray Old Age Ghosts
The gray ghost is a Sim who died from old age, the quickest way to do this is to use the
testingcheatsenabled cheat. Hold shift and left click on the active Sim and choose Trigger
Age Transition this will make them die from old age and their ghost will be gray and
sparkly.
Quickest way to bring someone back from the dead.
Make sure you kill Sims with the highest needs possible. (Some like Drowning are
unavoidable) Every time they spawn from their grave they will have the exact same needs as
they had when they died. After you kill someone, bring a new person with 0 skills to the
lot. Then use the testingcheatsenabled cheat code and set their needs to static. This will
essentially make them stand still doing nothing. After a few game hours (maybe days tops)
they will get their first opportunity, since they have 0 skills and the lot has a ghost on
it, the opportunity will be to resurrect the dead. Place the tomb stone in their inventory
and head to the science lab to bring them back! With this method it should only take 10
minutes to create a ghost, assuming you already have a dead sim.
How do I bring someone back from the dead?
There is an opportunity where the science lab asks you to bring a dead loved one to the
science building for testing. Your living Sim must at least be friends with the dead Sim
you want to resurrect. Completing this will give you a playable ghost. You will not get
the resurrection opportunity phone call right away when a Sim dies, it may take a long
time if your living Sim has skills. The living Sim having skills adds skill related
opportunities to the pool of potential opportunities. If your Sim knows nothing, and has
never left the house, you will get the ghost opportunity first. Make sure you have no
opportunities and deny any that you receive until you get the one asking you to bring your
loved one to the lab.
What if they died on another lot?
Go to the graveyard, walk into the mausoleum, and choose Manage Graves all unplaced stones
will be there. You can place them in the yard, or take them home in the Sims inventory. Be
warned, some people have reported a bug with this where they cannot get the tombstone of
someone who died.
infomation has been copyed from
http://asilee.com/2009/09/09/the-sims-3-death-and-ghost-guide/