Control a 5 volt water pump with an Arduino I want to control (only on/off) a small water pump (5&nbsp;V, 200&nbsp;mA max.) with my Arduino. 

I read something about NPN transistor, because the output pins of the Arduino can't supply enough current to the pump. However, I don't know how to choose the right transistor and how to connect it with my Arduino and my water pump.
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.