Hello,
may i send a PWM output into a DC motor or it is dangerous for the motor?
Does the PWM break a DC motor?
Yes, you can send PWM to a DC motor, but not directly from the processor unless your motor is very tiny and draws less than 40mA - you need a drive transistor or H-bridge.
I've realized this circuit:
External power source, in my circuit, is 9V but i hope i can put 12V...
Did you intend something similar with transistor?
If your answer is yes, what about H-bridge?
PS: Sorry for my bad english...
External power source, in my circuit, is 9V but i hope i can put 12V
But you show 3V.
The BC547 has a maximum Ice of only around 100mA, I think, which isn't a lot of current for a motor.
Do you have a spec for the motor, or have you measured the current when it is opearting?
I show 3V because i've done a cut&paste of a image in internet 
I do not have any information of DC motor: i got it from a toy... I only know it is little (in dimension...).
I do not think it require 100mA but this night i'll measure the current when it is operating; thanks a lot.
I think you'd be very surprised. A cheap little $1 hobby motor weighing half a gram can require 500mA no load up to a couple of amps in a stall. And those low voltage/high current motors are a pain to drive with off the shelp gear.
Get yourself a motor driver and save the headaches.
You'll need a much larger transistor for most cheapie motors.
If you are looking for offboard, something like this one:
http://www.solarbotics.com/products/k_cmd/
Otherwise,
http://www.hvwtech.com/products_view.asp?ProductID=991