The width of the pulses in the PWM controls the amount of average current in the fan. But the BC108 is very old and had a maximum allowed collector current of only 100mA. Do you have a tiny fan?
What is Rb? A series base resistor? Its current should be 1/10th to 1/20th the collector current. With a +12V supply the datasheet of the LM358 shows that its typical output with with a 10mA load is +10.5V.
The datasheet shows that the typical output high current is 40mA so it is best to use Rb.