How to safely use a transistor to control fans
The first step is to determine the expected current that the fan will draw.  Let's call it 200 mA (just a wild guess on my part, but you should be able to find or measure it).
Next find the minimum beta of the transistor (I'll assume 50, just for example).  In order to pull 200 mA with a beta of 50, you'll need 200/50 = 4 mA of base current.
In order to get 4 mA of base current, divide the voltage drop from control to Vbe (if output of the control is 3.3 V and the Vbe is 0.6 V, then the drop is 3.3-0.6 = 2.7 V) by the base current.  2.7 V / 4 mA = 675 ohms.
Hope this helps and welcome to the site.