Control fan speed with photocell

Hi,
I have a DC 12V fan.
It has two wires.

I want the speed of the fan to be controlled by a photocell. Is this possible? And if so, how?

I have a circuit now. I'm using the 555 (intergrated circuit). I have the photocell and the fan connected on the other side. When I connect the battery, the fan works but as I move my finger up and down to change the light value received by the photocell, this does not affect the speed of the fan. However, there is a fluctuating noise coming from the fan as I do this.

Thank you for your help,
Val