At first,someone told me to design a car burglar alarm but I decided to make a slight revision to it. I decided to make one that instead of setting off the horn of the car it will set off a voice recorded message. But I don't know how to do this
I would suggest starting with what you have been asked for and treating voice messages as a separate module. There are a few chips around that do audio recording and playback (I think sparkfun sell some). Find one you like and then compare what it requires as a trigger to what your alarm supplies as an output and go from there.
And remember that going over and above what you are asked for may impress your teacher, but only if you get the basic bit that they asked for correct. There is no point adding extra bits unless your alarm is near perfect.