Use a MOSFET and a DC Motor

Hello, Firstly,I'm French and I'm my sorry for my bad English. This is my problem : I'm working on a DC-Motor and I need to control the speed. I'm using a MOSFET and a PWM DC-DC. When I test with a resistor,I measure a PWM voltage but with the DC Motor I get an other signal. My motor is connected to a vehicle body. I would like to get a PWM signal when I measure the voltage because for the moment I can't controlled the speed because voltage is always 12V. Thank you in advance for the answers.