/diy/ - Do It Yourself

I'm trying to amplify an arduino PWM signal using a mosfet.
I'm trying to control the speed of a small DC motor (R1 in the pic) but it isn't small enough to be driven by the arduino itself, it needs more power. So I'm trying to feed it with a 12V power supply but modulate that using a mosfet triggered by the arduino PWM signal. Pic related is my circuit.
It doesn't work because mosfet stays on instead of opening the circuit when the signal is low. I think it has to do with its capacitance. How do I fix it? I've tried putting a resistor in series with the gate and the ground so the mosfet could discharge through it but that didn't work.