My question is how to choose the frequency suitable for the PWM signal?
One engineering say me that low frequency can burn the motor and high frequency can burn the transistor
Is it right?
Well... No, better tell that engineer to "brush" up on DC motor theory
Too low a frequency is just converging towards DC (full power 100% of the time), which won't burn anything if selected for the motor (i.e. 12V for a 12V motor).
Too high a frequency will give greater loss, since the loss is in the transitions - and the higher the frequency, the higher the number of transitions/second.
PWM on DC motors is usually most efficient at frequencies around 500Hz to 5kHz (depending on the actual motor) and as such, will give an audible effect.
How I can choose the right frequency?
You could make a test set-up with a 555 and a power transistor and find the frequency which give the best combination of efficiency and the lowest possible level of sound with the motor in question.
Or you could just go with, say 2kHz and be done with it.