150mA to the fan. 12V.
The transistor Vbe is about 0.7V. The datasheet rating of 1.3V is a maximum at maximum base current.
To make sure a transistor is saturated, you should put in a base current between 1/20 and 1/10 of the collector current. So about 8 to 15mA. This is because gain of a transistor goes down when in saturation, so the gain at 15mA unsaturated is not an accurate assessment of what it is when saturated. In addition, you want the transistor overdriven so a different batch, temperature, or aging doesn't prevent saturation.
(5V - 0.7V)/10mA = 430 ohms
(5V - 0.7V)/15mA = 287 ohms
The output of an Arduino will not go all the way to 5V when you are drawing current from it, so I'd go with 330 ohms.
Then the Vce saturated should be around 0.1 to 0.2V.
BTW, 15mA is 0.015A. 0.15A is 150mA. The prefix "m" lower case or "milli" means one thousandths.