Control a 5 volt water pump with an Arduino
To use an NPN, connect a power rail of 5V to the pump's positive terminal, and from its negative terminal wire it to the transistor's collector pin. Wire emitter to GND, and place a >=220 ohms resistor between your arduino control pin and the transistor base. 

If instead using an N-channel MOSFET, wire load to drain, source to GND, and use a 4K7 pull-down on gate. 

Also add a diode, with its anode at the pump's negative terminal, and cathode (white ring) at the positive terminal.