How to change speed of dc motor using potentiometer?
LM317: 



To use your 10K pot you can use it for R2 and change R1 to 1.2K ohms. It will not work correctly without the fan connected because the load current will be too low for the LM317 to regulate. 

Adjustment range (with fan connected) should be 1.25 to around 12V, assuming at least 14V in. Check it doesn't get too hot- you might have to add a heatsink or bolt it to something- at least you have a fan to cool it.