iLelouch:
sry, i mean.. i have to make a serial communikation (rs232 ) between a computer and the arduino ide (seriell monitor),
This makes no sense because the Arduino Serial Monitor is a program running on the computer and there is no need for serial communication between the the computer and the Serial Monitor.
My guess is that you were not listening when the teacher gave out the assignment. If so you need to ask your teacher to explain it again.
...R