How can I use USB output (5V DC) to power a fan 12V DC load?
Before attempting the project, lets evaluate the power budget available to ensure the USB port responsible to supply power won't get damaged.

As you may know the power that a typical usb port can provide is \$P = V \times I\$ where V is 5V and I varies depending on how powerful the source is. It could go up to 3 amperes for very powerful socket USB charger. For PCs it wouldn't really be wise to extract any more then 1 ampere without protection to avoid a very costly motherboard repair for your PC.

To get 12V DC from 5V DC you need to use a power converter. In this case, a boost converter. But we all know the law of conservation of energy, the amount of power that goes into the converter must be equal to the amount of power that goes out. 

So in an equation
\$V_{in} \times I_{in} \times Efficiency_{convertor} = V_{out}*I_{out}\$

Typical boost converters on Amazon would given you an efficiency between .80 to .95 depending on design and load you apply. These boost converters feature a variable potentiometer that allows you to adjust the output voltage and get 12V DC.

So now you know you can get 12V DC from 5V DC but as you increased the voltage, the output current must reduce to respect conservation principle. So in reality you can extract very limited amount of power from your converter. Also remember boost converters aren't really happy when you apply zero load to their output, and this of course depends on how well they have been designed, the output voltage may increase to a large value.

If you have interest you may want to learn about designing a boost converter but that is a discussion for another day and has its own challenges. But do study boost converters online and try making one in a simulation software.