Press [Shift] + [Ctrl] + C (Shift + Ctrl + Windows Key + C for Vista) and that will display the console window, where you can write the cheats and then press enter and that particular cheat will be activated.

Some of the common cheats are:

Cheat Code Result CODE
Display most cheats help
1,000 more Simoleons Kaching
50,000 more Simoleons Motherlode
+2,500 lifetime happiness points to active Sim shazaam
Ignore the cost when buying a lot freerealestate
Set or clear traits modify traits
Add active Sim to current household add to household
Set age of the Sim set age [number]
Take the Sim back to Create-a-Sim edit in cas
Give selected Sim a career set career [career/level]
Click career building you work at to force an opportunity force opportunity
Click career building you work at to force an event force event
Click career building to display all events for the career consecutively force all events
Force a specific service Sim to appear force service sim [name]
Forces a neighbor to show up force visitor
Selected Sim knows every other Sim make me know everyone
Make several random friends for selected Sim make friends for me
Sets all mood/moodlets for everyone in the house to perfect; removes negative moodlets make happy
Set motives static or dynamic for entire household make motives [static or dynamic]
View career outfits and service uniforms [Note 1] unlockOutfits [on or off]
No limitations for placing/moving objects moveObjects [on or off]
Return Sims to safe and nuetral state at home resetSim [first name] [last name]
Force terrain adjustments regardless of objects, Sims, and other structures constrainFloorElevation [true or false]
Objects will not snap to slots while holding [Alt] disableSnappingToSlotsOnAlt[on or off]
Objects fade when camera gets close to them fadeObjects [on or off]
Toggle 45 degree angle object snap snapObjectsToAngle [true or false]
Toggle snap to grid for placed objects snapObjectsToGrid [true or false]
Show talk/thought balloons above Simís head hideHeadlineEffects [on or off]
Print random joke to console jokePlease
Llamas enabled message enablellamas [on or off]
Slow motion visuals; 0 is normal, 8 is slowest slowMotionViz [0-8]
Enable testing cheats testingCheatsenabled [true or false]
Toggle framerate display at top right fps [on or off]
Full screen fullscreen [on or off]
Quit the game quit