Multiplying the voltage of an output pin on an Arduino board
image selected at random from somewhere in the intertubes.  


ditto

Some people prefer MOSFETs


ditto

See also prior art

For a bigger motor you might use a BJT to turn on a heftier MOSFET


ditto

Important notes


Ensure you use a switching device that can comfortably handle more than whatever maximum current the specific motor will need. 
It is good practice to have a snubber diode across the motor to prevent it causing damage to the switching elements.
The ground of the motor's power source should be connected to the ground of the Arduino's power source (e.g. the header socket marked "GND")